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>

llvm-svn: 178410