[libc++][format] Fixes vector<bool> requirements.

Makes sure the formatter for the vector<bool>::reference is enabled
when only the header <vector> is included. Before this change it
required <vector> and <format> to be included. This violated the
requirements in the Standard.

Fixes: https://llvm.org/PR61314

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D149543