Attach Lazy ClangImporter Diagnostics as Notes

Clang importer diagnostics that are produced as a result of a reference
in Swift code are attached to as notes to the Sema produced diagnostic
that indicates the declaration is unavailable.

Ex: Notes about why a C function import failed are attached to
the error explaining that the symbol could not be found in scope.

lib/Serialization/Deserialization.cpp

@@ -6402,13 +6402,6 @@ void ModuleFile::loadAllMembers(Decl *container, uint64_t contextData) {
   }
 }
 
-void ModuleFile::diagnoseMissingNamedMember(const IterableDeclContext *IDC,
-                                            DeclName name) {
-  // TODO: Implement diagnostics for failed member lookups from module files.
-  llvm_unreachable(
-      "Missing member diangosis is not implemented for module files.");
-}
-
 static llvm::Error consumeErrorIfXRefNonLoadedModule(llvm::Error &&error) {
     // Missing module errors are most likely caused by an
     // implementation-only import hiding types and decls.