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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

It looks like ConstantFolding.cpp is missing the pragmas.

For host.cpp (and its caller ApplyHostFunctionHelper), it looks like there are also missing pragmas.

I think we need to fix those first.

Maybe @lntue can comment on how LIBC_COMPILER_HAS_STDC_FENV_ACCESS is supposed to work with hand-in-hand?

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