Fix test for bots where -std=c99 is the default

Amends 9791f25

This addresses the issue found by:
https://lab.llvm.org/buildbot/#/builders/144/builds/13023

Author: AaronBallman

clang/test/Sema/enum.c

@@ -129,6 +129,10 @@ typedef struct Color NewColor; // expected-error {{use of 'Color' with tag type
 #else
   _Static_assert(__has_extension(c_fixed_enum), "");
   _Static_assert(!__has_feature(c_fixed_enum), "");
+#if __STDC_VERSION__ < 201112L
+  // expected-warning@-3 {{'_Static_assert' is a C11 extension}}
+  // expected-warning@-3 {{'_Static_assert' is a C11 extension}}
+#endif
 #endif
 typedef enum : unsigned char { Pink, Black, Cyan } Color; // pre-c23-warning {{enumeration types with a fixed underlying type are a C23 extension}}