libstdc++: std::hash::operator is not constexpr

The overloads of std::hash::operator cannot be constexpr as
std::hash::operator, as defined libstdc++, are not constexpr either.

Apparently this is also what C++ standard specifies, though it would
be quite convenient that std::hash::operator would be defined as
constexpr.

Bug: 957519
Change-Id: I80b93e83cc5352dd1ba5ab4979c6051818dac82d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4372589
Commit-Queue: José Dapena Paz <jdapena@igalia.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1122458}

base/containers/id_map_unittest.cc

@@ -32,7 +32,7 @@ struct RepeatingKeyType {
 namespace std {
 template <>
 struct hash<::base::test::id_map::RepeatingKeyType> {
-  constexpr size_t operator()(
+  size_t operator()(
       const ::base::test::id_map::RepeatingKeyType& k) const noexcept {
     return std::hash<int>()(k.i);
   }