[libc][stdbit] Fix truncation err in CPP bit_ceil (#84683)

After #84657 was merged,
[buildbot](https://lab.llvm.org/buildbot/#/builders/250/builds/19808) is
reporting two errors for libc-x86_64-debian-gcc-fullbuild-dbg. This PR
addresses the truncation error for `CPP::bit_ceil<unsigned char>` and
`CPP::bit_ceil<unsigned short>`.

The errors are:

```
FAILED: projects/libc/src/stdbit/CMakeFiles/libc.src.stdbit.stdc_bit_ceil_uc.dir/stdc_bit_ceil_uc.cpp.o
/usr/bin/g++ -DLIBC_NAMESPACE=__llvm_libc_19_0_0_git -D_DEBUG -I/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/build/projects/libc/src/stdbit -I/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/stdbit -I/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc -isystem /home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/build/projects/libc/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-vir!
 tual-dtor
  -Wsuggest-override -Wno-comment -Wno-misleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -g -fpie -ffreestanding -fno-builtin -fno-exceptions -fno-lax-vector-conversions -fno-unwind-tables -fno-asynchronous-unwind-tables -fno-rtti -ftrivial-auto-var-init=pattern -Wall -Wextra -Werror -Wconversion -Wno-sign-conversion -Wimplicit-fallthrough -Wwrite-strings -Wextra-semi -DLIBC_COPT_PUBLIC_PACKAGING -std=c++17 -MD -MT projects/libc/src/stdbit/CMakeFiles/libc.src.stdbit.stdc_bit_ceil_uc.dir/stdc_bit_ceil_uc.cpp.o -MF projects/libc/src/stdbit/CMakeFiles/libc.src.stdbit.stdc_bit_ceil_uc.dir/stdc_bit_ceil_uc.cpp.o.d -o projects/libc/src/stdbit/CMakeFiles/libc.src.stdbit.stdc_bit_ceil_uc.dir/stdc_bit_ceil_uc.cpp.o -c /home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/stdbit/stdc_bit_ceil_uc.cpp
In file included from /home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/stdbit/stdc_bit_ceil_uc.cpp:11:
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/__support/CPP/bit.h: In instantiation of ‘constexpr __llvm_libc_19_0_0_git::cpp::enable_if_t<is_unsigned_v<T>, T> __llvm_libc_19_0_0_git::cpp::bit_ceil(T) [with T = unsigned char; enable_if_t<is_unsigned_v<T>, T> = unsigned char]’:
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/stdbit/stdc_bit_ceil_uc.cpp:17:23:   required from here
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/__support/CPP/bit.h:196:57: error: conversion from ‘unsigned int’ to ‘unsigned char’ may change value [-Werror=conversion]
  196 |   return static_cast<T>(T(1) << cpp::bit_width<T>(value - 1u));
      |                                                   ~~~~~~^~~~
cc1plus: all warnings being treated as errors
[138/466] Building CXX object projects/libc/src/stdbit/CMakeFiles/libc.src.stdbit.stdc_bit_ceil_us.dir/stdc_bit_ceil_us.cpp.o
FAILED: projects/libc/src/stdbit/CMakeFiles/libc.src.stdbit.stdc_bit_ceil_us.dir/stdc_bit_ceil_us.cpp.o
/usr/bin/g++ -DLIBC_NAMESPACE=__llvm_libc_19_0_0_git -D_DEBUG -I/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/build/projects/libc/src/stdbit -I/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/stdbit -I/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc -isystem /home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/build/projects/libc/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-vir!
 tual-dtor
  -Wsuggest-override -Wno-comment -Wno-misleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -g -fpie -ffreestanding -fno-builtin -fno-exceptions -fno-lax-vector-conversions -fno-unwind-tables -fno-asynchronous-unwind-tables -fno-rtti -ftrivial-auto-var-init=pattern -Wall -Wextra -Werror -Wconversion -Wno-sign-conversion -Wimplicit-fallthrough -Wwrite-strings -Wextra-semi -DLIBC_COPT_PUBLIC_PACKAGING -std=c++17 -MD -MT projects/libc/src/stdbit/CMakeFiles/libc.src.stdbit.stdc_bit_ceil_us.dir/stdc_bit_ceil_us.cpp.o -MF projects/libc/src/stdbit/CMakeFiles/libc.src.stdbit.stdc_bit_ceil_us.dir/stdc_bit_ceil_us.cpp.o.d -o projects/libc/src/stdbit/CMakeFiles/libc.src.stdbit.stdc_bit_ceil_us.dir/stdc_bit_ceil_us.cpp.o -c /home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/stdbit/stdc_bit_ceil_us.cpp
In file included from /home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/stdbit/stdc_bit_ceil_us.cpp:11:
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/__support/CPP/bit.h: In instantiation of ‘constexpr __llvm_libc_19_0_0_git::cpp::enable_if_t<is_unsigned_v<T>, T> __llvm_libc_19_0_0_git::cpp::bit_ceil(T) [with T = short unsigned int; enable_if_t<is_unsigned_v<T>, T> = short unsigned int]’:
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/stdbit/stdc_bit_ceil_us.cpp:17:23:   required from here
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/__support/CPP/bit.h:196:57: error: conversion from ‘unsigned int’ to ‘short unsigned int’ may change value [-Werror=conversion]
  196 |   return static_cast<T>(T(1) << cpp::bit_width<T>(value - 1u));
      |                                                   ~~~~~~^~~~
cc1plus: all warnings being treated as errors
```

0 files changed, 0 insertions, 0 deletions