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/front.pass.cpp

@@ -12,6 +12,10 @@
 // const charT& front() const;
 //       charT& front();
 
+#ifdef _LIBCPP_DEBUG2
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+#endif
+
 #include <string>
 #include <cassert>
 
@@ -42,4 +46,11 @@ int main()
     test(S("1234567890123456789012345678901234567890"));
     }
 #endif
+#ifdef _LIBCPP_DEBUG2
+    {
+        std::string s;
+        char c = s.front();
+        assert(false);
+    }
+#endif
 }