For now, disallow lifetime-extended temporaries with non-trivial (but

constexpr) destructors from being used in the values of constexpr
variables.

The standard rules here are unclear at best, so rejecting the
problematic cases seems prudent. Prior to this change, we would fail to
run the destructors for these temporaries, even if they had
side-effects, which is certainly not the right behavior.

llvm-svn: 373161