Un-XFAIL a test under new GCC version; the GCC bug has been fixed

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@328229 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/comparisons/constexpr_init.pass.cpp

@@ -9,7 +9,7 @@
 
 // UNSUPPORTED: c++98, c++03, c++11
 
-// XFAIL: gcc-7, gcc-8
+// XFAIL: gcc-7
 
 // <functional>