Fix Issue 22014 - Make core.stdc.stddef.wchar_t match C++ mangling

[kinke]

Just like we already do in core.stdc.stdint.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22014	normal	Wrong MSVC++ mangling of wchar_t

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#15916"

[kinke]

Blocked by apparently insufficient magic for these special enums, e.g.:

• for the underlying 16-bit wchar on Windows:

  core\stdc\wchar_.d(128): Error: cannot implicitly convert expression `65535` of type `int` to `__c_wchar_t`
  

• for the underlying 32-bit dchar on Linux:

  std/stdio.d(3248): Error: none of the overloads of template `std.utf.encode` are callable using argument types `!()(__c_wchar_t[1], immutable(dchar))`
  …
  std/utf.d(2510):                        `encode(Flag useReplacementDchar = No.useReplacementDchar)(out dchar[1] buf, dchar c)
  

[Herringway]

The same missing magic also blocks std::wstring from working in D (at least on windows, didn't test other OSes)