When elevating access along an inheritance path, initialize the computed

When elevating access along an inheritance path, initialize the computed
access to the (elevated) access of the accessed declaration, if applicable,
rather than plunking that access onto the end after we've calculated the
inheritance access.

Also, being a friend of a derived class gives you public access to its
members (subject to later modification by further inheritance);  it does
not simply ignore a single location of restricted inheritance.

Also, when computing the best unprivileged path to a subobject, preserve
the information that the worst path might be AS_none (forbidden) rather
than a minimum of AS_private.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@98899 91177308-0d34-0410-b5e6-96231b3b80d8