[Clang] Fix behavior of -ffp-model option when overriden

-ffp-model=strict -ffp-model=fast will still enable strict exception
handling behavior, therefore clang still emits constrained FP operations
in IR.

-ffp-model=fast -ffp-model=strict emits two warnings: one for strict
overriding fast, the other for strict overriding strict, which is
confusing.

Reviewed By: zahiraam

Differential Revision: https://reviews.llvm.org/D137618