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
Visit and serialize C++ fields by checking if a var template's context is a CXXRecordDecl in VisitVarTemplateDecl.
Depends on D158027
Reviewed By: dang
Differential Revision: https://reviews.llvm.org/D158029