SILGen: Support mutable consuming parameters.

Emit a box like we would for a local variable, and move the parameter value into the box, as
part of the prolog.

test/SILGen/consuming_parameter.swift

@@ -0,0 +1,29 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+
+func bar(_: String) {}
+
+// CHECK-LABEL: sil {{.*}} @${{.*}}3foo
+func foo(y: consuming String, z: String) -> () -> String {
+    // CHECK: bb0(%0 : @owned $String, %1 : @guaranteed $String):
+    // CHECK:   [[BOX:%.*]] = alloc_box ${ var String }
+    // CHECK:   [[BOX0:%.*]] = mark_uninitialized [var] [[BOX]]
+    // CHECK:   [[BOX1:%.*]] = begin_borrow [lexical] [[BOX0]]
+    // CHECK:   [[Y:%.*]] = project_box [[BOX1]]
+    // CHECK:   store %0 to [init] [[Y]]
+
+    // CHECK:   [[YCAPTURE:%.*]] = copy_value [[BOX1]]
+    // CHECK:   partial_apply {{.*}} {{%.*}}([[YCAPTURE]])
+    let r = { y }
+
+    // CHECK:   [[ZCOPY:%.*]] = copy_value %1
+    // CHECK:   [[YACCESS:%.*]] = begin_access [modify] [unknown] [[Y]]
+    // CHECK:   assign [[ZCOPY]] to [[YACCESS]]
+    y = z
+
+    // CHECK:   [[YACCESS:%.*]] = begin_access [read] [unknown] [[Y]]
+    // CHECK:   [[YVAL:%.*]] = load [copy] [[YACCESS]]
+    // CHECK:   apply {{%.*}}([[YVAL]]
+    bar(y)
+
+    return r
+}