c++: Add fixed test [PR91506]

Pre-r11-557 we issued a bogus

  error: parameter may not have variably modified type 'double [x]'

but now we compile this, as we should.

gcc/testsuite/ChangeLog:

	PR c++/91506
	* g++.dg/init/array60.C: New test.

gcc/testsuite/g++.dg/init/array60.C

@@ -0,0 +1,8 @@
+// PR c++/91506
+
+double
+test(int *arr, int x)
+{
+    double ret(double(arr[x]) + 1);
+    return ret;
+}