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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@kuhar I've added a few more tests on iterator traits.  Also, I've corrected a bug where `value_type` was a const value type in `const_iterator`.  IIUC, `value_type` is always a non-const value type.

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