[clang] [Clang] Honor '#pragma STDC FENV_ROUND' in __builtin_store_half/halff (PR #173821)

[Wenju He via cfe-commits]

wenju-he wrote:

> It can still matter if backend passes run, and this isn't disabling passes

Done, added -disable-llvm-passes flag.

https://github.com/llvm/llvm-project/pull/173821