[Sema] Don't look for the instantiation of a local extern decl in a different
dependent context that the one we are instantiating, otherwise there will be an assertion.

rdar://15464547

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195828 91177308-0d34-0410-b5e6-96231b3b80d8