druntime: Fix core.stdc.stdint.(u)int64_t on 64-bit macOS etc.

[kinke]

• Use it instead of newly introduced, LDC-specific core.stdc.config.c(pp)_(u)int64_t.
• Cherry-pick Walter's upstream commit replacing the magic structs in core.stdc.config by magic enums (supporting implicit conversion from integer literals) [as well as adding cpp_{size,ptrdiff}_t].
• Add a clarifying comment wrt. DMD/LDC mangling difference on 64-bit OS X and use version(LDC) guards.
• Adapt dmd-testsuite accordingly.

[JohanEngelen]

Related: #2650

I find this mangling situation very unfortunate :( Sorry if asking the same question again, I forgot the answer. Is it now that from 2.079 onwards it is necessary to use different D types for C++ interop with DMD and LDC? Is there any way we can prevent that? (either convince DMD to do what we think is sane, or change LDC to do what DMD thinks is sane?)

[kinke]

A short summary:

• No different D types need to be used, now that there's proper cpp_(u)long, cpp_{size,ptrdiff}_t and (u)int64_t.
• With DMD >= 2.079, you can use native D (u)long to represent C++ (unsigned) long long and (u)int64_t, although that's a bad idea as it's not portable (e.g., won't work on Linux x86_64).
• With LDC, it continues to represent C++ (unsigned) long and {size,ptrdiff}_t, maintaining backwards compatibility and, if you don't care about 32-bit OSX, not requiring an extra import and the ugly cpp_ prefixes for your extern(C++) declarations making ample use of size_t (due to perfect match on all other platforms for both {size,ptrdiff}_t).
• I tried to convince Walter multiple times already, e.g., just yesterday in change size_t to uint for OSX 64 bit dlang/dmd#8249.

[dnadlinger]

With LDC, it continues to represent C++ (unsigned) long and {size,ptrdiff}_t

What does "it" refer to here? D [u]long is not of the same width as C++ [unsigned] long on all platforms, e.g. Windows/MSVC.

[kinke]

The above summary is all 64-bit macOS specific, except for the parts where I mentioned 32-bit macOS and 'other platforms'. So 'it = D (u)long on 64-bit macOS and thus D {size,ptrdiff}_t on 64-bit macOS`.

[kinke]

[On Windows/MSVC, D (u)long is C++ (unsigned) long long. But so is C++ size_t (for Win64), ergo no issues.]

[Oh and D long is C++ long on all other 64-bit POSIX platforms (and C++ long long on 32-bit POSIX incl. 32-bit Mac).]