libstdc++: bitset _GLIBCXX_ASSERTIONS op[] fixes

C++11 forbids a compound statement, as seen in the definition
of __glibcxx_assert(), in a constexpr function. This patch
open-codes the assertion in `bitset<>::operator[] const` for
C++11 to fix a failure in `g++.old-deja/g++.martin/bitset1.C`.

Also, it adds `{ dg-do compile }` in another test to suppress
a spurious UNRESOLVED complaint.

libstdc++-v3/ChangeLog:
	* include/std/bitset (operator[]() const): Customize bounds
	check for C++11 case.
	* testsuite/20_util/bitset/access/subscript_const_neg.cc:
	Suppress UNRESOLVED complaint.

libstdc++-v3/include/std/bitset

@@ -1298,8 +1298,16 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       _GLIBCXX_CONSTEXPR bool
       operator[](size_t __position) const
       {
+#if __cplusplus != 201103L
 	__glibcxx_assert(__position < _Nb);
 	return _Unchecked_test(__position);
+#elif defined(_GLIBCXX_ASSERTIONS)
+	// C++11 forbids a compound stmt in a constexpr function.
+	return __position < _Nb ? _Unchecked_test(__position)
+	  : (__builtin_trap(), false);
+#else
+	return _Unchecked_test(__position);
+#endif
       }
       ///@}
 

libstdc++-v3/testsuite/20_util/bitset/access/subscript_const_neg.cc

@@ -1,3 +1,4 @@
+// { dg-do compile }
 #include <bitset>
 
 void test_const_subscript_assignment()