[clang] [Driver] Clean up fp-contract handling in clang driver (PR #99723)

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> This is a refreshed version of #91271. Given how long it's been since I updated that PR, it seemed better to just start fresh.

May be #91271 should be closed then,
In `Driver/fp-contract.c`, all the `//RUN` lines where you removed the `-Werror` should now have `//WARN` check lines.

https://github.com/llvm/llvm-project/pull/99723