[clang] [clang][DebugInfo] Make fwd decl call site debug info consistent for methods (PR #222263)

[Jeremy Morse via cfe-commits]

https://github.com/jmorse approved this pull request.

This makes sense to me; please add a comment as to why there are two paths for DISubprogram production in the code added though.

Would it make sense to push this logic into EmitFunctionDecl, or is that too complicated?

https://github.com/llvm/llvm-project/pull/222263