implement exception_wrapper::is_compatible_with<Ex> directly

Summary: Rather than implementing it via `with_exception`, which is implemented in terms of `handle`, which is complex. Works now that exact casting can always be done without `catch`.

Reviewed By: ot, luciang

Differential Revision: D27987235

fbshipit-source-id: 822b0fe168e3b55c506b88810e13573b87c381c7

folly/ExceptionWrapper-inl.h

@@ -445,7 +445,7 @@ inline folly::fbstring exception_wrapper::class_name() const {
 
 template <class Ex>
 inline bool exception_wrapper::is_compatible_with() const noexcept {
-  return with_exception([](Ex const&) {});
+  return get_exception<Ex>();
 }
 
 [[noreturn]] inline void exception_wrapper::throw_exception() const {