[Bazel] Use dynamic workspace root determination

The `clang:ast` and `clang:builtin_headers_gen` targets currently use hardcoded `external/llvm-project`
paths to access generated headers.

With bzlmod this path becomes dependent on the module name, module version and module extension,
so we need a more dynamic approach.

Does not affect the WORKSPACE build.

Reviewed By: GMNGeoffrey, #bazel_build

Differential Revision: https://reviews.llvm.org/D137007