swift234

Sema: Add DeclContext::isGenericTypeContext()

Once nested generic parameter lists are properly chained, we need a
way of checking if we're inside a generic type context that's
distinct from just checking if we have a generic type signature
available.

This distinguishes between these two cases:

class A<T> {
  // generic signature
  func method() -> T { // <T> A<T> -> () -> T
  }
}

func f<T>() {
  class A {
    // no generic signature
    func method() -> T { // A -> () -> T
    }
  }
}