clang: Treat ieee mode as the default for denormal-fp-math

The IR hasn't switched the default yet, so explicitly add the ieee
attributes.

I'm still not really sure how the target default denormal mode should
interact with -fno-unsafe-math-optimizations. The target may have
selected the default mode to be non-IEEE based on the flags or based
on its true behavior, but we don't know which is the case. Since the
only users of a non-IEEE mode without a flag still support IEEE mode,
just reset to IEEE.