Separate `-dump-clang-diagnostics` into two features.

Motivation

-dump-clang-diagnostics currently does two somewhat orthogonal things:

1. emits ad-hoc "remarks" on things ClangImporter is doing to stderr
2. emits clang diagnostics through both clang's and swift's diagnostic engine

Proposed solution

We should extract 1) into actual remarks, and enable them with -Rclang-importer (or whatever option https://github.com/swiftlang/swift/pull/87726 ends up adding).

Alternatives considered

-dump-clang-diagnostics may be left with just 2), or also enabling -Rclang-importer by default to keep the same behavior. I personally lean towards the former to avoid tangling orthogonal concepts, but I don't have a strong opinion.

Additional information

No response