[Clang] Allow the value of unroll count to be zero in `#pragma GCC unroll`...

[Clang] Allow the value of unroll count to be zero  in `#pragma GCC unroll` and `#pragma unroll` (#88666)

Fixes https://github.com/llvm/llvm-project/issues/88624

GCC allows the value of loop unroll count to be zero, and the values of
0 and 1 block any unrolling of the loop. This PR aims to make clang
keeps the same behavior with GCC.
https://gcc.gnu.org/onlinedocs/gcc/Loop-Specific-Pragmas.html



---------

Signed-off-by: yronglin <yronglin777@gmail.com>