[clang] Ast importer visitors (PR #138838)

[Balázs Kéri via cfe-commits]

balazske wrote:

After change #141104 it is possible to compile and link the code without problems.
There are some things to fix in this patch (with naming and formatting rules and code correctness), I would like better if the code would look like in PR #138845. Additionally tests are needed for all of the new `Decl` and `Expr` nodes. 

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