Fix broken unit tests added in cherry pick of D157757

c23 is still c2x on stable/20230725
rdar://116859121

Author: ian-twilightcoder

clang/test/Headers/stddef.c

@@ -1,6 +1,6 @@
 // RUN: %clang_cc1 -fsyntax-only -verify=c99 -std=c99 %s
 // RUN: %clang_cc1 -fsyntax-only -verify=c11 -std=c11 %s
-// RUN: %clang_cc1 -fsyntax-only -verify=c23 -std=c23 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c23 -std=c2x %s
 
 struct astruct { char member; };
 

clang/test/Headers/stddefneeds.c

@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify=c99 -std=c99 %s
-// RUN: %clang_cc1 -fsyntax-only -verify=c23 -std=c23 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c23 -std=c2x %s
 
 // Use C99 to verify that __need_ can be used to get types that wouldn't normally be available.