[clang] [llvm] [LLVM][Support] Enforce proper instantiations of `Registry<T>` (PR #185141)

[Saleem Abdulrasool via cfe-commits]

compnerd wrote:

> > Does it have measured perf implications that we should consider?
> 
> I haven't measured, however, accessing `Registry<T>` shouldn't be in a hot-path, so I think there is no major impact. For build-time, `Registry<T>` itself is a quite simple template, and I don't think so many TUs instantiate it with the same parameters, so I guess it's not significant.

Sorry about not being clear, I meant compile time performance implications, i.e. does this change the compile times/sizes meaningfully for MSVC, clang, or gcc? That is fair, it it is a pretty simple member, so it shouldn't be too expensive to materialise, but the problem is on the linking side where the deduplication needs to occur.

https://github.com/llvm/llvm-project/pull/185141