Fix parse tree of DROP TRANSFORM and COMMENT ON TRANSFORM

The plain C string language name needs to be wrapped in makeString() so
that the parse tree is copyable.  This is detectable by
-DCOPY_PARSE_PLAN_TREES.  Add a test case for the COMMENT case.

Also make the quoting in the error messages more consistent.

discovered by Tom Lane

src/backend/commands/functioncmds.c

@@ -1867,7 +1867,7 @@ CreateTransform(CreateTransformStmt *stmt)
 		if (!stmt->replace)
 			ereport(ERROR,
 					(errcode(ERRCODE_DUPLICATE_OBJECT),
-					 errmsg("transform for type %s language %s already exists",
+					 errmsg("transform for type %s language \"%s\" already exists",
 							format_type_be(typeid),
 							stmt->lang)));