Fix -fno-unsafe-math-optimizations behavior (#89473)

This changes the handling of -fno-unsafe-fp-math to stop having that
option imply -ftrapping-math. In gcc, -fno-unsafe-math-optimizations
sets -ftrapping-math, but that dependency is based on the fact the
-ftrapping-math is enabled by default in gcc. Because clang does not
enable -ftrapping-math by default, there is no reason for
-fno-unsafe-math-optimizations to set it.

On the other hand, -funsafe-math-optimizations continues to imply
-fno-trapping-math because this option necessarily disables strict
exception semantics.

This fixes https://github.com/llvm/llvm-project/issues/87523