UBSan: Don't diagnose inf/nan conversions between floating-point types. It's...

UBSan: Don't diagnose inf/nan conversions between floating-point types. It's far from clear whether these have undefined behavior, and these checks are helping no-one. Keep the double->float overflow warnings, though, since those are useful in practice, even though it's unclear whether such operations have defined behavior.

llvm-svn: 178194