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
(cherry picked from commit fedf24c)

Author: augusto2112

lib/ClangImporter/ImportType.cpp

@@ -3429,7 +3429,9 @@ ImportedType ClangImporter::Implementation::importAccessorParamsAndReturnType(
   // FIXME: Duplicated from importMethodParamsAndReturnType.
   DeclContext *origDC = importDeclContextOf(property,
                                             property->getDeclContext());
-  assert(origDC);
+  if (!origDC)
+    return {Type(), false};
+
   auto fieldType = isGetter ? clangDecl->getReturnType()
                             : clangDecl->getParamDecl(0)->getType();