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
Reintroduce D153557 with fix for use-after-free from f4de606ef271 and minor changes.
Reviewed By: dang
Differential Revision: https://reviews.llvm.org/D157007
