testsuite: constexpr-diag1.C and implicit constexpr

This test doesn't break as expected with implicit constexpr.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-diag1.C: Add -fno-implicit-constexpr.

gcc/testsuite/g++.dg/cpp1y/constexpr-diag1.C

 // PR c++/111272
 // { dg-do compile { target c++14 } }
-// { dg-options "-Werror=invalid-constexpr" }
+// { dg-options "-Werror=invalid-constexpr -fno-implicit-constexpr" }
 // { dg-prune-output "some warnings being treated as errors" }
 
 struct Jam