Worked around GCC bug 56480. Explicit specialization in a different namespace.

Summary: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56480

Reviewers: EricWF

Reviewed By: EricWF

Differential Revision: https://reviews.llvm.org/D31273

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@298581 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/optional/optional.hash/hash.pass.cpp

@@ -22,11 +22,15 @@
 struct A {};
 struct B {};
 
+namespace std {
+
 template <>
-struct std::hash<B> {
+struct hash<B> {
   size_t operator()(B const&) { return 0; }
 };
 
+}
+
 int main()
 {
     using std::optional;