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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178194 91177308-0d34-0410-b5e6-96231b3b80d8