[Sema] Add defensive guard for implicit use of self in closure check

Add assetion along with defensive guard to avoid crashing in release
build. Hopefully, sourcekitd Stress Tester find a reproducer for the
crash.

rdar://problem/47895109
(cherry picked from commit ce30a72)

Author: rintaro

lib/Sema/MiscDiagnostics.cpp

@@ -1625,10 +1625,20 @@ static void diagnoseImplicitSelfUseInClosure(TypeChecker &TC, const Expr *E,
     /// Return true if this is an implicit reference to self.
     static bool isImplicitSelfUse(Expr *E) {
       auto *DRE = dyn_cast<DeclRefExpr>(E);
-      return DRE && DRE->isImplicit() && isa<VarDecl>(DRE->getDecl()) &&
-             cast<VarDecl>(DRE->getDecl())->isSelfParameter() &&
-             // Metatype self captures don't extend the lifetime of an object.
-             !DRE->getType()->is<MetatypeType>();
+
+      if (!DRE || !DRE->isImplicit() || !isa<VarDecl>(DRE->getDecl()) ||
+          !cast<VarDecl>(DRE->getDecl())->isSelfParameter())
+        return false;
+
+      // Defensive check for type. If the expression doesn't have type here, it
+      // should have been diagnosed somewhere else.
+      Type ty = DRE->getType();
+      assert(ty && "Implicit self parameter ref without type");
+      if (!ty)
+        return false;
+
+      // Metatype self captures don't extend the lifetime of an object.
+      return !ty->is<MetatypeType>();
     }
 
     /// Return true if this is a closure expression that will require "self."