Add all Decls found through FindExternalLexicalDecls

to the DeclContext.  This fulfils the contract that
we make with Clang by returning ELR_AlreadyLoaded.

This is a little aggressive in that it does not allow
the ASTImporter to import the child decls with any
lexical parent other than the Decl that reported them
as children.

<rdar://problem/13517713>

llvm-svn: 181498