Mark a struct definition in an objc container with the TopLevelDeclInObjCContainer bit.

Fixes accurately getting a cursor inside an objc container containing a struct definition,
from a PCH/preamble file.

rdar://12584613

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173811 91177308-0d34-0410-b5e6-96231b3b80d8