Re-commit r259942 (reverted in r260053) with a different workaround for the MSVC bug.

This fixes undefined behavior in C++14 due to the size of the object being
deleted being different from sizeof(dynamic type) when it is allocated with
trailing objects.

MSVC seems to have several bugs around using-declarations changing the access
of a member inherited from a base class, so use forwarding functions instead of
using-declarations to make TrailingObjects::operator delete accessible where
desired.

llvm-svn: 260180