libstdc++: Constrain bitset(const CharT*) constructor [PR121046]

libstdc++: Constrain bitset(const CharT*) constructor [PR121046]

Asking std::is_constructible_v<std::bitset<1>, NonTrivial*> gives an
error, rather than answering the query. The problem is that the
constructor for std::bitset("010101") is not constrained to only accept
pointers to char-like types, and for the second parameter (which has a
default argument) std::basic_string_view<CharT> gets instantiated. If
the type is not char-like then that has undefined behaviour, and might
trigger a static_assert to fail in the body of std::basic_string_view.

We can fix it by constraining that constructor using the requirements
for char-like types from [strings.general] p1.  I've submitted LWG 4294
and proposed making this change in the standard.

libstdc++-v3/ChangeLog:

	PR libstdc++/121046
	* include/std/bitset (bitset(const CharT*, ...)): Add
	constraints on CharT type.
	* testsuite/23_containers/bitset/lwg4294.cc: New test.