[clang] Add clang warning if fp exception functions are called without appropriate flags/pragmas (PR #199009)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

@eugeneepshteyn Any update on flang?

@maarcosrmz were you able to try building flang like I suggested?

https://github.com/llvm/llvm-project/pull/199009