Make the Twine pretty-printer work with GDB 7.11

Apparently ::NodeKind is sometimes part of the name in GDB.
Without this patch I get the following error message from GDB:
`Unhandled NodeKind llvm::Twine::NodeKind::EmptyKind`.

Patch by Alexander Richardson!

Differential Revision: https://reviews.llvm.org/D32795

llvm-svn: 304675