libstdc++: port bitwise relocatable away from is_trivial

In preparation for the deprecation of is_trivial (P3247R2).
"bitwise relocation" (or "trivial relocation" à la P1144/P2786)
doesn't need the full-fledged notion of triviality, just checking for a
trivial move constructor and a trivial destructor is sufficient.

libstdc++-v3/ChangeLog:

	* include/bits/stl_uninitialized.h: Amended the
	__is_bitwise_relocatable type trait.

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