swift234

Dramatically improve the diagnostics when a store is invalid, by taking 
into account accesibility, assignments to self in a non-mutating
method (consistently), recursive components of an lvalue that makes it 
non-settable, etc.  Now we tell you what the *problem* was, instead of
just whining.

This fixes:
<rdar://problem/19370429> QoI: fixit to add "mutating" when assigning to a member of self in a struct
<rdar://problem/17632908> QoI: Modifying struct member in non-mutating function produces difficult to understand error message

in their full generality.



Swift SVN r28867