TestHelpers.h pulls few pieces from LLVM libc's unittest framework
which aren't available on platforms like Fuchsia which use their own
unittest framework. So, by moving FPExceptMatcher to a different file
we can exclude LLVM libc specific pieces in a cleaner way.
In a later pass, it might make more sense to rename TestHelpers.h also
to FPMatcher.h. That way, we can make macros like EXPECT_FP_EQ to be
equivalent to EXPECT_EQ on platforms like Fuchsia.