c++: Add test for DR 188.

gcc/testsuite/ChangeLog:

	DR 188
	* g++.dg/DRs/dr188.C: New test.

gcc/testsuite/g++.dg/DRs/dr188.C

@@ -0,0 +1,7 @@
+// DR 188
+// { dg-do compile { target c++11 } }
+// From [diff.expr]p5.
+
+char arr[100];
+// Yields 100 in C++ and sizeof(char*) in C.
+static_assert (sizeof (0, arr) == 100, "");