Other implementations of the symbol graph format use zero-based indices
for source locations, which causes problems when combined with clang's
current one-based indices. This commit sets ExtractAPI's symbol graph
output to use zero-based indices to align with other implementations.
rdar://107639783
Add has_template template, DeclarationFragmentBuilder functions, and tests for class templates, specializations/partial specs, and concepts.
Depends on D157007
Reviewed By: dang
Differential Revision: https://reviews.llvm.org/D157076