Fix PR6648 by not creating a temporary with the type of a

Fix PR6648 by not creating a temporary with the type of a
CXXExprWithTemporaries.

Not emitting the expression as an aggregate might be the right thing to do,
but is orthogonal. Emitting it as an scalar expression will still try to
create a temporary for the incomplete type of the CXXExprWithTemporaries and
fail.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99134 91177308-0d34-0410-b5e6-96231b3b80d8