Move an exception_ptr in exceptionStr

Summary: [Folly] Move an `exception_ptr` in `exceptionStr`, which should be ever-so-slightly cheaper than copying it since that may avoid atomic refcount operations. It's not much cheaper overall because the throw/catch machinery remains expensive.

Reviewed By: ot, luciang

Differential Revision: D22529858

fbshipit-source-id: 7e5d4ec5c1ca601a766c63133ae759651ae7be85

folly/ExceptionString.cpp

@@ -16,6 +16,8 @@
 
 #include <folly/ExceptionString.h>
 
+#include <utility>
+
 namespace folly {
 
 /**
@@ -40,7 +42,7 @@ fbstring exceptionStr(std::exception_ptr ep) {
   return catch_exception(
       [&]() -> fbstring {
         return catch_exception<std::exception const&>(
-            [&]() -> fbstring { std::rethrow_exception(ep); },
+            [&]() -> fbstring { std::rethrow_exception(std::move(ep)); },
             [](auto&& e) { return exceptionStr(e); });
       },
       []() -> fbstring { return "<unknown exception>"; });