Added a testcase that checks that fairly complicated
structures are parsed safely by the Objective-C runtime.

Also made some modifications to the way we parse structs
in the runtime to avoid mis-parsing @ followed by the name
of the next field.

<rdar://problem/18887634>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@221643 91177308-0d34-0410-b5e6-96231b3b80d8