Check if decl context returned by importDeclContextOf is null

I've seen a crash due to importDeclContextOf returning a null decl
context in importAccessorParamsAndReturnType. Most other call sites of
importDeclContextOf check the returned value, so add a check in
importAccessorParamsAndReturnType too.

rdar://127847162

Author: augusto2112

lib/ClangImporter/ImportType.cpp

@@ -3430,6 +3430,9 @@ ImportedType ClangImporter::Implementation::importAccessorParamsAndReturnType(
   DeclContext *origDC = importDeclContextOf(property,
                                             property->getDeclContext());
   assert(origDC);
+  if (!origDC)
+    return {Type(), false};
+
   auto fieldType = isGetter ? clangDecl->getReturnType()
                             : clangDecl->getParamDecl(0)->getType();