Update for newer Clang

[CraigMacomber]

Update to support templight built with current version of clang (tested at 1d39f2e5adc9fb5e0d5ec45a5ebddf1534398ff9).

Required updating InstantiationKindStrings to match CodeSynthesisContext::SynthesisKind in Sema.h from Clang (I was getting corrupted strings from indexing off the end of that array: I assumed this was the correct fix, and it seems to work now, but I have not confirmed the data is fully accurate)

Also included is some small refactoring to deduplicate the two copies of InstantiationKindStrings.

This likely will cause incorrect output when used with older versions of clang, so I'm not sure if you want this change upstream, an/or if you want to include some version disclaimers.