Emit an error message instead of crashing when dereferencing an incomplete pointer type.

If the ASM statement is dereferencing an incomplete pointer type, issue an error
instead of crashing.
<rdar://problem/12700799>


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177915 91177308-0d34-0410-b5e6-96231b3b80d8