Add missed test coverage

This amends b67d557bd3 to add the test
coverage for those changes.

clang/test/Preprocessor/c2y.c

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c2y %s
+// expected-no-diagnostics
+
+// FIXME: Set this to test the correct value once that value is set by WG14.
+static_assert(__STDC_VERSION__ > 202311L, "Incorrect __STDC_VERSION__");
+