Prior to emission of `.d` and `.swiftdeps` outputs. In implicit builds such dependencies are registered during the construction of the corresponding Clang module by the ClangImporter's built-in Compiler Instance. In explicit builds, since we load pre-built PCMs directly, we do not get to do so. So instead, manually register all `.h` inputs of Clang module dependnecies.
Resolves rdar://121354886