[Sema] Support -testable-import-module to load transitive non-public dependencies

When using the -testable-import-module argument to insert a testable
import, there's no ImportDecl on which to show the diagnostics when
loading transitive dependencies. Clean up the logic to still load
dependencies in such a case.

Author: xymus

lib/Sema/ImportResolution.cpp

@@ -326,8 +326,11 @@ void ImportResolver::bindImport(UnboundImport &&I) {
 
   // Load more dependencies for testable imports.
   if (I.import.options.contains(ImportFlags::Testable)) {
+    SourceLoc diagLoc;
+    if (ID) diagLoc = ID.get()->getStartLoc();
+
     for (auto file: M->getFiles())
-      file->loadDependenciesForTestable(ID.get()->getStartLoc());
+      file->loadDependenciesForTestable(diagLoc);
   }
 
   auto topLevelModule = I.getTopLevelModule(M, SF);

lib/Serialization/SerializedModuleLoader.cpp

@@ -1479,10 +1479,9 @@ void SerializedASTFile::loadDependenciesForTestable(SourceLoc diagLoc) const {
     File.loadDependenciesForFileContext(this, diagLoc, /*forTestable=*/true);
 
   if (status != serialization::Status::Valid) {
-    if (diagLoc)
-      serialization::diagnoseSerializedASTLoadFailureTransitive(
-          getASTContext(), diagLoc, status, &File,
-          getParentModule()->getName(), /*forTestable*/true);
+    serialization::diagnoseSerializedASTLoadFailureTransitive(
+        getASTContext(), diagLoc, status, &File,
+        getParentModule()->getName(), /*forTestable*/true);
   }
 }