[clang] Ast importer visitors (PR #138838)

[Balázs Kéri via cfe-commits]

balazske wrote:

I think that `ASTImporter` must be moved into a new component (directory in "clang/lib") to fix this problem. `AST` is already dependency of `Sema` so `Sema` can not be added to `AST` as dependency (but could be added to `ASTImporter` if it would be a new component and `Sema` would not need `ASTImporter`). Or the code of some concept related classes must be moved but this looks more difficult.

https://github.com/llvm/llvm-project/pull/138838