Allow C99 hexfloats in C++0x mode. This change resolves the standards

collision between C99 hexfloats and C++0x user-defined literals by
giving C99 hexfloats precedence. Also, warning about user-defined
literals that conflict with hexfloats and those that have names that
are reserved by the implementation. Fixes <rdar://problem/9940194>.

llvm-svn: 138839