swift234

Introduce lvalue open-existential expressions and start using them.

Place OpenExistentialExprs for references to lvalue subscripts or properties
(in protocol extensions) via existentials just outside of the member
or subscript reference, rather than far outside the expression. This
gives us a tighter bound on the open-existential expressions without
introducing the post-pass I was threatening.

OpenExistentialExprs just outside of lvalue member/subscript are
themselves lvalues. Resurrect John's OpenOpaqueExistentialComponent to
handle the opening of a (materialized) existential lvalues as an
lvalue path component. This has the nice effect of codifying the
formal access rules for opened existentials as well as handling inout
on opened existentials appropriately.

Big thanks to John for talking through the model with me and leaving
dead code around for me to use.

Swift SVN r27105