[clang] [clang] Use constant rounding mode for floating literals (PR #90877)

[Serge Pavlov via cfe-commits]

spavloff wrote:

> The changes should come with a release note so that users know about the behavioral change, but otherwise LGTM.

Pragma FENV_ROUND is still officially unsupported, compiler issues a warning about this. It is https://github.com/llvm/llvm-project/pull/89617 that makes FENV_ROUND supported, it requires release note.

https://github.com/llvm/llvm-project/pull/90877