When emitting a compound literal of POD type, continue to emit a

When emitting a compound literal of POD type, continue to emit a
separate aggregate temporary and then memcpy it over to the
destination. This fixes a regression I introduced with r133235, where
the compound literal on the RHS of an assignment makes use of the
structure on the LHS of the assignment.

I'm deeply suspicious of AggExprEmitter::VisitBinAssign()'s
optimization where it emits the RHS of an aggregate assignment
directly into the LHS lvalue without checking whether there is any
aliasing between the LHS/RHS. However, I'm not in a position to
revisit this now.

Big thanks to Eli for finding the regression!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133261 91177308-0d34-0410-b5e6-96231b3b80d8