[libc++][format] Updates feature-test macros.

During the discussion on the SG-10 mailinglist regarding the format
feature-test macros voted in during the last plenary it turns out libc++
can't mark the format feature-test macro as implemented.

According to
  https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#__cpp_lib_format
the not yet implemented paper
  P1361R2 Integration of chrono with text formatting
affects the feature test macro.

Note that P1361R2 doesn't mention the feature-test macro nor is there an
LWG-issue to address the issue. The reporter of the issue didn't recall
where this requirement exactly has been decided.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D133271