Fixed the way ClangASTImporter deports types from
ASTContexts that will not stay around.  Before, we
did this in a very half-hearted way.  Now we maintain
work queues of all Decls that need to be completed
before the source ASTContext can go away; we then
expunge their origins completely.

<rdar://problem/13511875>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178410 91177308-0d34-0410-b5e6-96231b3b80d8