Introduce proper facilities to render token spellings using the diagnostic
formatter.

Replaces most of the hard-coded diagnostic messages related to expected tokens,
which all shared the same semantics but had to be multiply defined due to
variations in token order or quote marks.

The associated parser changes are largely mechanical but they expose
commonality in whole chunks of the parser that can now be factored away.

This commit uses C++11 typed enums along with a speculative legacy fallback
until the transition is complete.

Requires corresponding changes in LLVM r197895.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197972 91177308-0d34-0410-b5e6-96231b3b80d8