[PATCH 0/4] MR6886: include: Enable format attribute on Clang in MSVC mode.

[Jacek Caban (＠jacek)]

The main motivation is to enable it for debug traces too, but for that we need !6852 merged first.

Clang is too strict about `%I` using the same type as `size_t`, which is `int` on 32-bit Windows targets. Ideally, it would be fixed in Clang, but we already have a hack for it in mingw mode, so I just extended it for now.

Changing type of `LONG_PTR` is pretty invasive, especially for C++ code (I'd expect it to break @rbernon's libc++ builds). We could probably guard it with `__cplusplus`. I restricted it even further with `__WINESRC__` under the assumption that external header users are less likely to care about this particular warning problem than about underlying type in general.