libstdc++: port away from is_trivial in string classes

In preparation for the deprecation of is_trivial (P3247R2), stop using
it from std::string_view. Also, add the same detection to std::string
(described in [strings.general]/2).

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h: Add a static_assert on the
	char-like type.
	* include/std/string_view: Port away from is_trivial.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>