[libc++][format] Fixes formatting vector<bool>

Formatting a const qualified vector<bool> fails to work on libc++. This
is due to our non-conforming type for vector<bool>::const_reference. The
type is a __bit_const_reference<vector> instead of a bool. (This is
fixed in ABI v2.)

This fixes this formatter and enables the test written for it.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D144279