Add a SIL test case for a dead alloc_stack with debug variable.

Author: atrick

test/DebugInfo/debug_value_addr.swift

@@ -22,3 +22,17 @@ func test<T>(_ t : T) {
   let a = S(a: t)
   a.foo()
 }
+
+func use<T>(_ t : T) {}
+
+// CHECK-SIL: sil hidden @$s16debug_value_addr11GenericSelfV1xACyxGx_tcfC : $@convention(method) <T> (@in T, @thin GenericSelf<T>.Type) -> GenericSelf<T> {
+// CHECK-SIL: bb0(%0 : $*T, %1 : $@thin GenericSelf<T>.Type):
+// CHECK-SIL-NEXT:  alloc_stack [lexical] $GenericSelf<T>, var, name "self", implicit, loc {{.*}}
+// CHECK-SIL-NEXT:  debug_value %0 : $*T, let, name "x", argno 1, expr op_deref, loc {{.*}}
+struct GenericSelf<T> {
+  init(x: T) {
+    // 'self' is a valid debug variable here even though there is
+    // nothing to initialize (the dead alloc_stack cannot be removed).
+    use(x)
+  }
+}