When performing a trivial copy of a C++ type, we must be careful not

When performing a trivial copy of a C++ type, we must be careful not
to overwrite objects that might have been allocated into the type's
tail padding.  This patch is missing some potential optimizations where
the destination is provably a complete object, but it's necessary for
correctness.

Patch by Jonathan Sauer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162254 91177308-0d34-0410-b5e6-96231b3b80d8