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

[Matheus Izvekov via cfe-commits]

mizvekov 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.

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