[libc] Add compile tests for each public header (#122527)

This adds a test that consists of compiling `#include <...>`,
pretty much alone, for each public header file in each different
language mode (`-std=...` compiler switch) with -Werror and many
warnings enabled.

There are several headers that have bugs when used alone, and
many more headers that have bugs in certain language modes.  So
for now, compiling the new tests is gated on the cmake switch
-DLLVM_LIBC_BUILD_HEADER_TESTS=ON.  When all the bugs are fixed,
the switch will be removed so future regressions don't land.

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