Allow parens within implicit member chains

[hamishknight]

We don't currently allow parens in the middle of an unresolved member chain:

struct S {
  static var foo: S { S() }
  var bar: S { S() }
}

let _: S = (.foo)     // works
let _: S = .foo.bar   // works
let _: S = (.foo).bar // error: cannot infer contextual base in reference to member 'foo'

However it's not clear this was an intentional design choice, as written there would be no other way to infer the base of .foo. We ought to consider lifting this restriction.