Better protocol conformance and substitution map printing

[huonw]

This stops recursive conformances from printing forever, and replaces them with something like:

protocol P { associatedtype A: P }
struct X: P { typealias A = X }
/*
(normal_conformance type=X protocol=P
  (assoc_type req=A type=X)
  (normal_conformance type=X protocol=P (details printed above)))
*/

(Also, notice that the above says (assoc_type req=A type=X) with the type desugared to X, whereas previously it would print the sugared version X.A because of the typealias.)

This also makes substitution map printed lisp-style and indent-awarely... nice for specialised conformances:

protocol P {}
struct X: P {}
class Super<T: P>: P {}
class Sub: Super<X> {}
/*
(inherited_conformance type=Sub protocol=P
  (specialized_conformance type=Super<X> protocol=P
    (substitution_map generic_signature=<T where T : P>
      (substitution T -> X)
      (conformance type=T
        (normal_conformance type=X protocol=P)))
    (normal_conformance type=Super<T> protocol=P)))
*/

And, lastly, minimises the size of substitution maps inside expressions (which occur on decl refs pointing to something generic):

func generic<T: Hashable>(_: T) {}
let _: (Int) -> () = generic
/*
...
(declref_expr type='(Int) -> ()' ... [with (substitution_map generic_signature=<T where T : Hashable> (substitution T -> Int))] function_ref=unapplied))
*/

Fixes rdar://problem/40074968.

[huonw]

@swift-ci please smoke test

[huonw]

@swift-ci please smoke test

[huonw]

@swift-ci please smoke test macOS

[huonw]

@swift-ci please smoke test

[huonw]

@swift-ci please smoke test

[slavapestov]

Excellent!