[libc++][format] Fixes invalid usage of m type.

The m type in a range formatter may only be used when a pair or a tuple
with two elements is used. This was not correctly validated as reported
in llvm.org/PR60995.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D145309

(cherry picked from commit 347a65a1)