swift234

reapply r12727 (passing an SGFContext down to emitGetAccessor from
emitRValueForDecl when we have it), now with a testcase.  We formerly 
compiled this:

protocol P {}
var lp : P { get: }

func f() -> P {
  return lp
}

into:

sil @_TF3let1fFT_PS_1P_ : $@thin (@out P) -> () {
bb0(%0 : $*P):
  // function_ref let.lp.getter : let.P
  %1 = function_ref @_TF3letg2lpPS_1P_ : $@thin (@out P) -> () // user: %3
  %2 = alloc_stack $P                             // users: %5, %4, %3
  %3 = apply %1(%2#1) : $@thin (@out P) -> ()
  copy_addr [take] %2#1 to [initialization] %0 : $*P // id: %4
  dealloc_stack %2#0 : $*@local_storage P         // id: %5
  %6 = tuple ()                                   // user: %7
  return %6 : $()                                 // id: %7
}


we now avoid the copy:

sil @_TF3let1fFT_PS_1P_ : $@thin (@out P) -> () {
bb0(%0 : $*P):
  // function_ref let.lp.getter : let.P
  %1 = function_ref @_TF3letg2lpPS_1P_ : $@thin (@out P) -> () // user: %2
  %2 = apply %1(%0) : $@thin (@out P) -> ()
  %3 = tuple ()                                   // user: %4
  return %3 : $()                                 // id: %4
}



Swift SVN r12748