Avoid -Wcatch-value warning in testsuite

	* testsuite/20_util/variant/run.cc: Catch exception by reference to
	prevent -Wcatch-value warning.

From-SVN: r270527

libstdc++-v3/ChangeLog

 2019-04-24  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/20_util/variant/run.cc: Catch exception by reference to
+	prevent -Wcatch-value warning.
+
 	* include/std/variant (__variant_construct): Use template parameter
 	type instead of equivalent decltype-specifier.
 	(_Move_ctor_base<false, Types...>::_Move_ctor_base(_Move_ctor_base&&)):

libstdc++-v3/testsuite/20_util/variant/run.cc

@@ -286,7 +286,7 @@ void emplace()
       Vector::allocator_type::set_limit(0);
       vv.emplace<2>(1, 1);
       VERIFY(false);
-    } catch (__gnu_cxx::forced_error) {
+    } catch (const __gnu_cxx::forced_error&) {
     }
     VERIFY(vv.valueless_by_exception());
   }