[clang] [llvm] Disentangle DISubprogram ODR type method uniquing from metadata uniquing (PR #217042)

[Orlando Cazalet-Hyams via cfe-commits]

OCHyams wrote:

> I think we can use the scope identifier since the scope having one is a prerequisite for the ODR uniquing

Done, open for review. As mentioned in the previous comment this is "NFC" for a selfhost clang thinLTO build with https://github.com/llvm/llvm-project/pull/222263 applied 

https://github.com/llvm/llvm-project/pull/217042