[clang] [InstallAPI] Hookup Input files & basic ASTVisitor (PR #82552)

[Fangrui Song via cfe-commits]

MaskRay wrote:

Hi, I believe this patch introduced a library layering issue.
https://llvm.org/docs/CodingStandards.html#library-layering

Since clangFrontend depends on clangInstallAPI, clangInstallAPI cannot include include/clang/Frontend header files.
CMake doesn't have good support for detecting library layering issues, but the unofficial build system Bazel can be used:

```sh
cd utils/bazel/llvm-project-overlay
bazel-6.3.2 build --config=generic_clang @llvm-project//clang
```

https://github.com/llvm/llvm-project/pull/82552