Debug mode for string. This commit also marks the first time libc++ debug-mode has found a bug (found one in regex). Had to play with extern templates a bit to get this to work since string is heavily used within libc++.dylib.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189114 91177308-0d34-0410-b5e6-96231b3b80d8

test/strings/basic.string/string.access/index.pass.cpp

@@ -12,6 +12,10 @@
 // const_reference operator[](size_type pos) const;
 //       reference operator[](size_type pos);
 
+#ifdef _LIBCPP_DEBUG2
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+#endif
+
 #include <string>
 #include <cassert>
 
@@ -47,4 +51,13 @@ int main()
     assert(s2[0] == '\0');
     }
 #endif
+#ifdef _LIBCPP_DEBUG2
+    {
+        std::string s;
+        char c = s[0];
+        assert(c == '\0');
+        c = s[1];
+        assert(false);
+    }
+#endif
 }