<mutex> : std::mutex::mutex is not constexpr

This is mentioned in other issues, but I wanted a bug report tracking this explicitly. For the curious see #946.

`std::mutex::mutex` is `constexpr` as of c++11, but even in c++17 MSVC will not support it.
Minimal repro:
https://godbolt.org/z/5Yc56hEG7
```c++
#include <mutex>

struct S {
    constexpr S() noexcept = default;
    std::mutex m_mutex;
};

static constexpr S s{};
```