[Typechecker] allow @autoclosure closure to be a typealias

[theblixguy]

This PR adds the ability for the @autoclosure closure to be a type alias.

Example:

class Foo {
  typealias FooClosure = () -> String
  func fooFunction(closure: @autoclosure FooClosure) {} // ok
}

Resolves SR-2688.

[theblixguy]

cc @jrose-apple

[CodaFi]

Looks good. Just one small request.

[xedin]

@swift-ci please smoke test and merge

[theblixguy]

@xedin can you invoke the CI again? had to fix a crash related to passing null to dyn_cast.

[xedin]

@swift-ci please smoke test

[theblixguy]

Still crashing 🤔 Assertion failed: (Val && "isa<> used on a null pointer")

import Metal
class Bar {
  func barFunction(closure: @escaping MTLDeviceNotificationHandler) {}
}

Let me investigate why this is happening, it seems to be crashing when type checking @escaping

[theblixguy]

All good now, I have ran a validation test locally to ensure everything's ok. @xedin can you invoke the CI one more time? thank you!

[xedin]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 7e12ae56edda5893f2ef4b927f20229be291c340

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 7e12ae56edda5893f2ef4b927f20229be291c340