swift234

fix:
<rdar://problem/16264989> property not mutable in closure inside of its willSet
and:
<rdar://problem/16826319> willSet immutability behavior is incorrect

by changing how we handle immutability of an observing property within its willSet.
Instead of trying to model it as an rvalue, just model it as an lvalue (which it is)
and diagnose the problem with a warning in MiscDiagnostics.



Swift SVN r18184