[C++] Fix the compiler errors reported by GCC 7

[0xFireWolf]

Abstract:

This PR fixes the compiler errors in src/FlatHashSet.h and src/FlatHashMap.h introduced in the previous PR #3694.

Details:

template <typename Key,
          typename Hash = typename std::unordered_set<Key>::hasher,
          typename Equal = typename std::unordered_set<Key>::key_equal,
          typename Allocator = typename std::unordered_set<Key>::allocator_type>
using FlatHashSet = std::unordered_set<Key, Hash, Equal, Allocator>;

The existing type definition of FlatHashSet/Map is problematic if Key is not natively hashable (i.e. No specialization of std::hash<Key> available, so std::unordered_set<Key> cannot be specialized and therefore the member type hasher does not exist), resulting in errors when the runtime library is being compiled by GCC 7 or Clang with GCC installation path set to GCC 7. You may find the error message produced by the Conan CI pipeline at here: conan-io/conan-center-index#12902 (comment).

Since the runtime library now targets C++17 which is supported by GCC 7, it would be great to fix this incompatibility. (To be honest, I was somewhat surprised by GCC 8+ that do not treat this as an error...)

Thank you.