[libc++][format] Adds integer formatter.

Implements the formatter for all fundamental integer types
(except `char`, `wchar_t`, and `bool`).
[format.formatter.spec]/2.3
For each charT, for each cv-unqualified arithmetic type ArithmeticT other
than char, wchar_t, char8_t, char16_t, or char32_t, a specialization
```
  template<> struct formatter<ArithmeticT, charT>;
```
This removes the stub implemented in D96664.

As an extension it adds partial support for 128-bit integer types.

Implements parts of:
- P0645 Text Formatting
- P1652 Printf corner cases in std::format

Completes:
- LWG-3248 #b, #B, #o, #x, and #X presentation types misformat negative numbers

Reviewed By: #libc, ldionne, vitaut

Differential Revision: https://reviews.llvm.org/D103433