Revert D998590

Summary:
- It looks like Xcode 5.0.2 / clang 500.2.79 changed this behavior. The
prior behavior was seen against Xcode 5.0.1 / clang 500.2.75. Blerg.
If we really have to support different Xcode flavors we can come up
with a real solution for this, but for now just take advantage of the
fact that clang and gcc seem to agree on this.

Test Plan: - make check on Mac OS X

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D1066272

Blame Revision: D998590

folly/test/ScopeGuardTest.cpp

@@ -127,11 +127,7 @@ TEST(ScopeGuard, GuardException) {
       throw std::runtime_error("destructors should never throw!");
     });
   },
-#if FOLLY_USE_LIBCPP
-  "terminate called throwing an exception"
-#else
-  "destructors should never throw"
-#endif
+  "destructors should never throw!"
   );
 }