[index] Update indexing to handle CXXDeductionGuideDecls properly

CXXDeductionGuideDecls can't be referenced so there's no need to output a symbol occurrence for them.
Also handle DeducedTemplateSpecializationTypeLocs in the TypeIndexer so we don't miss the symbol occurrences of the corresponding template decls.

Patch by Nathan Hawes!

Differential Revision: https://reviews.llvm.org/D36641

llvm-svn: 310933