Add extension warning test

Author: Sirraide

clang/test/SemaCXX/cxx2c-expansion-stmt-ext-warn.cpp

@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 %s -std=c++2c -fsyntax-only -verify=cxx26 -Wpre-c++26-compat
+// RUN: %clang_cc1 %s -std=c++23 -fsyntax-only -verify=cxx23
+
+void f() {
+  template for (auto _ : {1}) { // cxx23-warning {{expansion statements are a C++2c extension}} \
+                                // cxx26-warning {{expansion statements are incompatible with C++ standards before C++2c}}
+  }
+}