libstdc++: Support constexpr formatting for vector<bool>::reference.

This specialization was made constexpr as part of P3391R2, "constexpr
format", and was missed in r17-1162-g42f9bf2e3c56ae.

libstdc++-v3/ChangeLog:

	* include/std/vector (formatter<_Bit_reference, _CharT>::format)
	[__glibcxx_constexpr_format]: Declare as constexpr.
	* testsuite/23_containers/vector/bool/format.cc
	[__glibcxx_constexpr_format]: Run test suite at compile
	time.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>