[Typechecker] Use getCalledValue() when checking ignored expressions

[theblixguy]

Instead of extracting the function declaration from the DeclRefExpr, use getCalledValue() instead. Otherwise, we throw the expression resolves to an unused function diagnostic for:

struct Foo {
  @discardableResult
  func bar() -> () -> Void { return {} }
}

Foo().bar() // error: expression resolves to an unused function

because we're not able to get back the function declaration in order to determine if it's been annotated with @discardableResult or not.

Fixes SR-12271

[theblixguy]

@swift-ci please smoke test

[DougGregor]

Thanks!