swift234

Rework how existential member references (i.e., calls to protocol methods)
to non-@mutating methods work in the AST: now the base expression is 
always computed as an rvalue, instead of computing them as an lvalue.  The 
optimization that we were accidentally getting before is now explicitly 
modeled, and the non-optimized case is now handled by standard temporary 
emission in SILGen instead of with MaterializeExpr.  The upshot of this 
carefully choreographed step is that there is no change in generated code (!).

Archetype member references still need to be switched over to this new
scheme (at which point materializeexpr is dead), and the optimization 
needs to be replicated for 'let' bases (at which point arguments 
becoming 'let' is only gated on debug info).



Swift SVN r12120