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

[Orlando Cazalet-Hyams via cfe-commits]

OCHyams wrote:

(I was away for a few weeks)

With https://github.com/llvm/llvm-project/pull/222263 this patch (with some modifications I alluded to above but haven't shared yet) produces an identical clang binary built with thinLTO as without the patch

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