Implement access declarations. Most of the work here is parsing them, which

Implement access declarations.  Most of the work here is parsing them, which
is difficult because they're so terribly, terribly ambiguous.

We implement access declarations in terms of using declarations, which is
quite reasonable.  However, we should really persist the access/using
distinction in the AST and use the appropriate name in diagnostics.  This
isn't a priority, so I'll just file a PR and hope someone else does it. :)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@91095 91177308-0d34-0410-b5e6-96231b3b80d8