Fixup 7457e2c by adding a switch case

This fixups 7457e2c as I forgot one
switch to update, which then broke this build bot:
https://lab.llvm.org/buildbot/#/builders/57/builds/33272

Author: steakhal

clang/unittests/AST/MatchVerifier.h

@@ -116,6 +116,10 @@ MatchVerifier<NodeType>::match(const std::string &Code,
     Args.push_back("-std=c++20");
     FileName = "input.cc";
     break;
+  case Lang_CXX23:
+    Args.push_back("-std=c++23");
+    FileName = "input.cc";
+    break;
   case Lang_OpenCL:
     Args.push_back("-cl-no-stdinc");
     FileName = "input.cl";