[clang] [clang][ExtractAPI] Add ability to create multiple symbol graphs (PR #86676)

[via cfe-commits]

https://github.com/QuietMisdreavus requested changes to this pull request.

Looks mostly good, though i would have preferred having multiple commits to wade through rather than having to spend 2.5 hours walking through it in person :wink: There are some comments that came out of that chat that i'd like to see addressed or investigated.

https://github.com/llvm/llvm-project/pull/86676