Some embedded platforms do not wish to support the C library functionality
for handling wchar_t because they have no use for it. It makes sense for
libc++ to work properly on those platforms, so this commit adds a carve-out
of functionality for wchar_t.
Unfortunately, unlike some other carve-outs (e.g. random device), this
patch touches several parts of the library. However, despite the wide
impact of this patch, I still think it is important to support this
configuration since it makes it much simpler to port libc++ to some
embedded platforms.
Differential Revision: https://reviews.llvm.org/D111265
Implements the formatter for all fundamental integer types.
[format.formatter.spec]/2.1
The specializations
```
template<> struct formatter<char, char>;
template<> struct formatter<char, wchar_t>;
template<> struct formatter<wchar_t, wchar_t>;
```
This removes the stub implemented in D96664.
Implements parts of:
- P0645 Text Formatting
Reviewed By: #libc, ldionne
Differential Revision: https://reviews.llvm.org/D103466
Louis Dionne