[clang] [clang-tools-extra] [clang] Improve nested name specifier AST representation (PR #147835)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

Yeah, all except one user of `getLocalSourceRange` expected the behavior to be what it is now.

I propose adding a note about this in the documentation, add a new helper which does what `getLocalSourceRange` used to do, and migrate the single existing user to use that.

https://github.com/llvm/llvm-project/pull/147835