Actually test the module header

Author: AaronBallman

clang/test/Modules/builtin-headers.mm

@@ -17,6 +17,7 @@
 @import _Builtin_stdarg;
 @import _Builtin_stdatomic;
 @import _Builtin_stdbool;
+@import _Builtin_stdcountof;
 @import _Builtin_stddef;
 @import _Builtin_stdint;
 @import _Builtin_stdnoreturn;