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

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

This seems to have caused the lib AMD GPU buildbot to break:
https://lab.llvm.org/staging/#/builders/11/builds/2190 and the mentioned #81788 did not fix it for the AMD bot.
@jhuber6 (or someone else) can you please take a look at this?
Thanks!

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