Implement C++11 [lex.ext]p10 for string and character literals: a ud-suffix not

starting with an underscore is ill-formed.

Since this rule rejects programs that were using <inttypes.h>'s macros, recover
from this error by treating the ud-suffix as a separate preprocessing-token,
with a DefaultError ExtWarn. The approach of treating such cases as two tokens
is under discussion for standardization, but is in any case a conforming
extension and allows existing codebases to keep building while the committee
makes up its mind.

Reword the warning on the definition of literal operators not starting with
underscores (which are, strangely, legal) to more explicitly state that such
operators can't be called by literals. Remove the special-case diagnostic for
hexfloats, since it was both triggering in the wrong cases and incorrect.

llvm-svn: 152287