Right now we drop the char_traits template argument, which presumes that
string<_CharT, _Traits> and string<_CharT> are interchangeable.
Reviewed By: Mordante, #libc, Quuxplusone
Differential Revision: https://reviews.llvm.org/D112017
Implements the formatter for all string types.
[format.formatter.spec]/2.2
For each charT, the string type specializations
```
template<> struct formatter<charT*, charT>;
template<> struct formatter<const charT*, charT>;
template<size_t N> struct formatter<const charT[N], charT>;
template<class traits, class Allocator>
struct formatter<basic_string<charT, traits, Allocator>, charT>;
template<class traits>
struct formatter<basic_string_view<charT, traits>, charT>;
```
This removes the stub implemented in D96664.
Implements parts of:
- P0645 Text Formatting
- P1868 width: clarifying units of width and precision in std::format
Reviewed By: #libc, ldionne, vitaut
Differential Revision: https://reviews.llvm.org/D103425
Mark de Wever