[clang] [clang-tools-extra] [llvm] [ADT] Refactor StringMap iterators (NFC) (PR #156137)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

Maybe we could add some tests with `is_same_v` to check that the types are as expected, e.g., that dereferencing const iterator give you the right const reference etc?

https://github.com/llvm/llvm-project/pull/156137