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.

llvm-svn: 256619