[clang] [NFC] [analyzer] Factor out SymbolManager::get<*> (PR #121781)

[Arseniy Zaostrovnykh via cfe-commits]

necto wrote:

@NagyDonat Here is the refactoring you suggested in https://github.com/llvm/llvm-project/pull/121551

I made sure `check-clang-analysis` passes after each commit, so I think reviewing it by commit should be easier than all at once.

https://github.com/llvm/llvm-project/pull/121781