User-defined literals: reject string and character UDLs in all places where the

User-defined literals: reject string and character UDLs in all places where the
grammar requires a string-literal and not a user-defined-string-literal. The
two constructs are still represented by the same TokenKind, in order to prevent
a combinatorial explosion of different kinds of token. A flag on Token tracks
whether a ud-suffix is present, in order to prevent clients from needing to look
at the token's spelling.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152098 91177308-0d34-0410-b5e6-96231b3b80d8