Limit workaround for Clang bug to __clang_major__ <= 7

The bug https://bugs.llvm.org/show_bug.cgi?id=33222 is now fixed on
Clang trunk, so the workaround won't be needed for Clang 8.0 and later.

	* include/std/variant (variant) [__clang__]: Limit workaround to
	Clang 7 and older.

From-SVN: r264271

libstdc++-v3/ChangeLog

+2018-09-13  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/std/variant (variant) [__clang__]: Limit workaround to
+	Clang 7 and older.
+
 2018-09-11  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/87278

libstdc++-v3/include/std/variant

@@ -1296,7 +1296,7 @@ namespace __variant
 
 #undef _VARIANT_RELATION_FUNCTION_TEMPLATE
 
-#ifdef __clang__
+#if defined(__clang__) && __clang_major__ <= 7
     public:
       using _Base::_M_u; // See https://bugs.llvm.org/show_bug.cgi?id=31852
     private: