Fix alignment issue in CodeGenFunction::PopCleanupBlock.

Fix alignment issue in CodeGenFunction::PopCleanupBlock.

It was copying an EHCleanupStack::Cleanup object into a
SmallVector<char>, with a comment saying that SmallVector's alignment is
always large enough. Unfortunately, that isn't actually true after
r162331 in 2012.

Expand the code (somewhat distastefully) to get a stack allocation with
a correct alignment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@256619 91177308-0d34-0410-b5e6-96231b3b80d8