[SIL] Ensure __owned arguments are @owned.

No matter what the default is, an __owned argument should end up being
@owned in SIL. Currently this is... hard to test, since arguments are
owned by default.

Part of rdar://problem/34668110.

Author: huonw

test/SILGen/owned.swift

@@ -6,11 +6,19 @@ class RefAggregate {}
 struct ValueAggregate { let x = RefAggregate() }
 
 // CHECK-LABEL: sil hidden @$S5owned0A10_arguments7trivial5value3refySin_AA14ValueAggregateVnAA03RefG0CntF : $@convention(thin) (Int, @owned ValueAggregate, @owned RefAggregate) -> () {
-func owned_arguments(trivial : __owned Int, value : __owned ValueAggregate, ref : __owned RefAggregate) {}
+func owned_arguments(trivial : __owned Int, value : __owned ValueAggregate, ref : __owned RefAggregate) {
+    let t = trivial
+    let v = value
+    let r = ref
+}
 
 struct Foo {
     var x: ValueAggregate
 
     // CHECK-LABEL: sil hidden @$S5owned3FooV20methodOwnedArguments7trivial5value3refySin_AA14ValueAggregateVnAA03RefJ0CntF : $@convention(method) (Int, @owned ValueAggregate, @owned RefAggregate, @guaranteed Foo) -> () {
-    func methodOwnedArguments(trivial : __owned Int, value : __owned ValueAggregate, ref : __owned RefAggregate) {}
+    func methodOwnedArguments(trivial : __owned Int, value : __owned ValueAggregate, ref : __owned RefAggregate) {
+        let t = trivial
+        let v = value
+        let r = ref
+    }
 }