[clang][Interp] Specify C standard in test case

Looks like some builders default to c99, which is a problem:
https://lab.llvm.org/buildbot/#/builders/139/builds/49750

Author: tbaederr

clang/test/AST/Interp/c.c

@@ -1,7 +1,7 @@
-// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify %s
-// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -pedantic -verify=pedantic-expected %s
-// RUN: %clang_cc1 -verify=ref %s
-// RUN: %clang_cc1 -pedantic -verify=pedantic-ref %s
+// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify -std=c11 %s
+// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -pedantic -verify=pedantic-expected -std=c11 %s
+// RUN: %clang_cc1 -verify=ref -std=c11 %s
+// RUN: %clang_cc1 -pedantic -verify=pedantic-ref -std=c11 %s
 
 /// expected-no-diagnostics