[clang] Implementation of '#pragma STDC FENV_ROUND' (PR #89617)

[Joshua Cranmer via cfe-commits]

https://github.com/jcranmer-intel commented:

Sorry for just thinking of this now, but we should also have tests for some of the builtins like `__builtin_fma` or `__builtin_sqrt`.

https://github.com/llvm/llvm-project/pull/89617