[libc++][format] Add basic_format_parse_context.

Implements parts of:
- P0645 Text Formatting

Depends on D92214

Reland with changes:
The format header will only be compiled if the compiler used has support
for concepts. This should fix the issues with the initial version.

Differential Revision: https://reviews.llvm.org/D93166

GitOrigin-RevId: 38ddeade65c50343a7f6a0daec57139b07ec7807