LWG 3158 issue addressed

[NathanSWard]

Description

Add a conditional explicit to std::tuple::tuple(std::allocator_arg_t, const Alloc&).
Fixes issue #69

Checklist

• I understand README.md. I also understand that acceptance of
  community PRs will be delayed until the test and CI systems are online.
• If this is a feature addition, that feature has been voted into the
  C++ Working Draft.
  I believe this is a bugfix, not a feature.
• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 .
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before CI is online, leave this unchecked for
  initial submission).
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
• These changes were written from scratch using only this repository and
  the C++ Working Draft as a reference (and any other cited standards).
  If they were derived from a project that's already listed in NOTICE.txt,
  that's fine, but please mention it. If they were derived from any other
  project (including Boost and libc++, which are not yet listed in
  NOTICE.txt), you must mention it here, so we can determine whether the
  license is compatible and what else needs to be done.

[NathanSWard]

Here is an example test case.

#include <tuple>
#include <memory>

struct implicit_t {
    implicit_t() = default;
};

struct explicit_t {
    explicit explicit_t() {}
};

int main() {
    std::tuple<implicit_t> tpl = {std::allocator_arg, std::allocator<implicit_t>{}};
    // std::tuple<explicit_t> tpl = {std::allocator_arg, std::allocator<explicit_t>{}}; <- should not compile
}

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[NathanSWard]

Note on the force push:
Had an unnecessary additional conjunction in the explicit's enable_if_t which I removed.

[StephanTLavavej]

Looks perfect. (And today I learned that a review comment saying /azp run doesn't do anything, it has to be a separate comment.)

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[CaseyCarter]

Looks good with test coverage.