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

[via llvm-commits]

lntue 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://lab.llvm.org/buildbot/#/builders/225/builds/31148

It should be fixed with https://github.com/llvm/llvm-project/pull/81788

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