ensure you cannot discard a local called self

previous checking for the expr being `self`
was checking for the decl being named `self`.

that meant you could do naughty things like:

```
let `self` = Self()
discard `self`
```

rdar://109376381
(cherry picked from commit 1c9ed33)

Author: kavon

lib/Sema/TypeCheckStmt.cpp

@@ -1314,13 +1314,14 @@ class StmtChecker : public StmtVisitor<StmtChecker, Stmt*> {
     if (!diagnosed) {
       bool isSelf = false;
       auto *checkE = DS->getSubExpr();
+      assert(fn->getImplicitSelfDecl() && "no self?");
 
       // Look through a load. Only expected if we're in an init.
       if (auto *load = dyn_cast<LoadExpr>(checkE))
           checkE = load->getSubExpr();
 
       if (auto DRE = dyn_cast<DeclRefExpr>(checkE))
-        isSelf = DRE->getDecl()->getName().isSimpleName("self");
+        isSelf = DRE->getDecl() == fn->getImplicitSelfDecl();
 
       if (!isSelf) {
         ctx.Diags

test/Sema/discard.swift

@@ -175,8 +175,20 @@ struct HasGenericNotStored<T>: ~Copyable { // expected-note 2{{arguments to gene
     let `self` = HasGenericNotStored<Int>()
     discard `self`
     // expected-error@-1 {{cannot convert value of type 'HasGenericNotStored<Int>' to expected discard type 'HasGenericNotStored<T>'}}
+    // expected-error@-2 {{you can only discard 'self'}}{{13-19=self}}
   }
 
   func identity(_ t: T) -> T { return t }
   deinit{}
 }
+
+struct Court: ~Copyable {
+  let x: Int
+
+  consuming func discard(_ other: consuming Court) {
+    let `self` = other
+    discard `self` // expected-error {{you can only discard 'self'}}{{13-19=self}}
+  }
+
+  deinit { print("deinit of \(self.x)") }
+}