Sema: Fix access control regression with generic type aliases

If a property had an inferred type that is a generic type alias,
we weren't checking the access level of the generic arguments.

This used to work in 4.1 because we didn't have a sugared
representation for generic type aliases, so we were checking
the substituted underlying type here. But now that NameAliasType
can store generic arguments, make sure we visit them here.

Fixes <rdar://problem/40188409>.

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -1408,8 +1408,7 @@ class TypeAccessScopeChecker : private TypeWalker, AccessScopeChecker {
       return Action::Stop;
 
     if (!CanonicalizeParentTypes) {
-      return isa<NameAliasType>(T.getPointer()) ? Action::SkipChildren
-                                                     : Action::Continue;
+      return Action::Continue;
     }
 
     Type nominalParentTy;

test/Sema/accessibility_typealias.swift

@@ -90,3 +90,13 @@ public var failNested2: (_ x: (main.ActuallyPrivate) -> Void) -> Void = { _ in }
 public func failTest(x: ActuallyPrivate) {} // expected-error {{cannot be declared public}}
 public func failTest2(x: main.ActuallyPrivate) {} // expected-error {{cannot be declared public}}
 
+// Property has an inferred type, public alias with
+// private generic parameter bound.
+public struct PublicGeneric<T> {}
+
+public typealias GenericAlias<T> = PublicGeneric<T>
+
+fileprivate func makeAValue() -> GenericAlias<ActuallyPrivate> { }
+
+public var cannotBePublic = makeAValue()
+// expected-error@-1 {{variable cannot be declared public because its type 'GenericAlias<ActuallyPrivate>' (aka 'PublicGeneric<ActuallyPrivate>') uses a private type}}