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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > Is there ANYTHING we can do to split this up? This is so huge that github is refusing to load half the changes.
> 
> I broke the commit down into others, now the first commit only has the AST and TreeTransform changes.

Can we split them up in reviews/do reviews as a stack if necessary? reviewing each individual 'commit' in the same review is also really difficult.

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