[clang] [clang] ASTContext: flesh out implementation of getCommonNNS (PR #131964)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

A few small comments, but comments on the function would be great.  Code itself looks fine as best I can tell.

https://github.com/llvm/llvm-project/pull/131964