[clang] Ast importer visitors (PR #138838)

[Balázs Kéri via cfe-commits]

balazske wrote:

I checked the code and it looks difficult to split the `ASTImporter` to a separate component. The problem is that `ExternalASTSource` is used for AST related things and uses `ASTImporter` too.
Another way to fix the problem is to move code of classes like `concepts::TypeRequirement` from `Sema` to `AST`. These seem to belong logically more to AST than Sema. I did not check how difficult would be to make these changes but it should be possible.

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