[clang][test] Try to fix constexpr-void-cast test

The test currenlty fails:
https://lab.llvm.org/buildbot/#/builders/139/builds/61628

because it emits a C11 warning when compiling as C. Try to fix that
be defining the C standard to use.

Author: tbaederr

clang/test/Sema/constexpr-void-cast.c

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -x c -fsyntax-only %s -verify=c
-// RUN: %clang_cc1 -x c -fsyntax-only %s -pedantic -verify=c-pedantic
+// RUN: %clang_cc1 -x c -fsyntax-only %s -verify=c -std=c11
+// RUN: %clang_cc1 -x c -fsyntax-only %s -pedantic -verify=c-pedantic -std=c11
 //
 // RUN: %clang_cc1 -x c++ -fsyntax-only %s -verify=cxx
 // RUN: %clang_cc1 -x c++ -fsyntax-only %s -pedantic -verify=cxx-pedantic