folly: avoid use of has_trivial_destructor in FBVector.h

folly: avoid use of has_trivial_destructor in FBVector.h

Summary:
This is required for gcc-4.8.x.
Use boost::has_trivial_destructor, not std::has_trivial_destructor.
The latter was never standard, and the now-std name (and the only one
supported in gcc-4.8.0) is std::is_trivially_destructible.

Test Plan:
fbmake runtests and then
run_unit_tests.sh with -r and again with -d

Reviewed By: njormrod@fb.com

FB internal diff: D923359

@override-unit-failures