[clang][AST] Encapsulate DeclarationNameLoc, NFCI

This change makes `DeclarationNameLoc` a proper class and refactors its
users to use getter methods instead of accessing the members directly.
The change also makes `DeclarationNameLoc` immutable (i.e., it cannot
be modified once constructed).

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D94596