Handle/assert DK_Remark in SourceMgrAdapter

We don't generate remarks for API Notes so no need to handle these for now.

(cherry picked from commit 3e3c07968dd669600354971757c7c075bdef9e65)

apple-llvm-split-commit: d004d20e0c4242d6b5a7dcf1157290c5edaddd03
apple-llvm-split-dir: clang/