[libc] [llvm] [libc] Add is_fixed_point type trait. (PR #81263)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

This is causing a failure in post submit

```
/home/sivachandra/libc-x86_64-debian/libc-x86_64-debian-dbg-runtimes-build/llvm-project/libc/src/__support/CPP/type_traits/is_fixed_point.h:30:23: error: use of undeclared identifier '_Fract'
   30 |       T, short fract, fract, long fract, unsigned short fract, unsigned fract,
      |                       ^
/home/sivachandra/libc-x86_64-debian/libc-x86_64-debian-dbg-runtimes-build/llvm-project/libc/include/llvm-libc-macros/stdfix-macros.h:21:15: note: expanded from macro 'fract'
   21 | #define fract _Fract
      |               ^
```
PTAL

https://github.com/llvm/llvm-project/pull/81263