PR33232: implement support for MSVC's __is_trivially_destructible trait.

Unlike the GCC-compatible __has_trivial_destructor trait, this one computes the
right answer rather than performing the quirky set of checks described in GCC's
documentation (https://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html).

MSVC also has a __has_trivial_destructor trait which is the same as its (and
now Clang's) __is_trivially_destructible trait; we might want to consider
changing the behavior of __has_trivial_destructor if we're targeting an MSVC
platform, but I'm not doing so for now.

While implementing this I found that we were incorrectly rejecting
__is_destructible queries on arrays of unknown bound of incomplete types; that
too is fixed, and I've added similar tests for other traits for good measure.

llvm-svn: 304376