[libc++][format] Mark LWG-issues as complete.

Most of the code has been implemented using the eel.is draft. It seems
some issues were inplemented but not marked as completed yet.

Note the wording of LWG-3372 has been implemented, but has been changed
in the current draft due to P2216, see D110494.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D112363