[libc++][format] Improves compile-time diagnostics.

Then a std-format-spec option is invalid for a type the compile-time
validation will detect its usage and issue a diagnostic. Before it
validated after parsing the entire std-format-spec, which meant the
diagnostic was less precise. It would be possible to do this validation
run-time but that has a performance overhead. When using the format
family of functions, this would be unneeded overhead; the validation was
done at run-time. For the vformat family it would give better
diagnostics.

To avoid paying what you don't use, it has been decided to aim for the
better performance. It's more likely users will use the format family of
functions.

Depends on D155264

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D155364