[clang] NFC: NNS getLocalSourceRange user cleanup (#163206)
This adds a note to the documentation of getLocalSourceRange, and cleans
up one of the users of this function which was changed in
https://github.com/llvm/llvm-project/pull/147835
This also removes `TypeLoc::getNonPrefixBeginLoc`, which was recently
introduced in the above patch, as that became unused.