Fix a bug where we will crash if we have a class "B" that is contained inside a class "A" in the DWARF where "A" is a forward declaration.

<rdar://problem/14673945>
<rdar://problem/15682781>



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@200985 91177308-0d34-0410-b5e6-96231b3b80d8