SILGen: Add missing weak_linked_attribute.swift test

I meant to check this in a while ago; it was part of
swiftlang#22834.

Author: slavapestov

test/SILGen/weak_linked_attribute.swift

@@ -0,0 +1,4 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+
+// CHECK-LABEL: sil [_weakLinked] [ossa] @$s21weak_linked_attribute0A8FunctionyyF : $@convention(thin) () -> ()
+@_weakLinked public func weakFunction() {}