PR12717: Clang supports hexadecimal floating-point literals in all language

PR12717: Clang supports hexadecimal floating-point literals in all language
modes. For languages other than C99/C11, this isn't quite a conforming
extension, and for C++11, it breaks some reasonable code containing
user-defined literals.

In languages which don't officially have hexfloats, pare back this extension
to only apply in cases where the token starts 0x and does not contain an
underscore. The extension is still not quite conforming, but it's a lot closer
now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158487 91177308-0d34-0410-b5e6-96231b3b80d8