[modules] Replace `-Wauto-import` with `-Rmodule-include-translation`.

Diagnostic for `-Wauto-import` shouldn't be a warning because it doesn't
represent a potential problem in code that should be fixed. And the
emitted fix-it is likely to trigger `-Watimport-in-framework-header`
which makes it challenging to have a warning-free codebase. But it is
still useful to see how include directives are translated into modular
imports and which module a header belongs to, that's why keep it as a remark.

Keep `-Wauto-import` for now to allow a gradual migration for codebases
using `-Wno-auto-import`, e.g., `-Weverything -Wno-auto-import`.

rdar://79594287

Differential Revision: https://reviews.llvm.org/D130138