[lldb] Change the representation of Swift enums

[adrian-prantl]

(Payload-carrying) Swift enums are a challenging datastructure to
support in LLDB; this patch makes an attempt to clean up a problem
with the previous implementation, namely the dynamic nature of the
children of an enum, which has, in the past, lead to caching bugs when
displaying the children of enums. The new scheme is as follows:

• The static value of an enum is its discriminator (its case).

• An enum has no static children.

• The synthtic value of an enum is its projected case's payload.

• A synthetic child provider for all Swift enums provides synthetic
  children, which are the children of the projected case's payload.

For example:

enum E {
  case a
  case b(Int, (Int, Int))
}

let a : E= .a
let b : E= .b(1, (2, 3))

The value of a is .a.
The value of b is .b.
The synthtic value of b is (1, (2, 3)).
The synthetic child 0 of b is 1.
The synthetic child 1 of b is (2, 3).
The number of (non-synthetic) children of b is 0.

The Swift Optional formatter behaves similarly.

Known Issues:

(FIXED!) - There is a bug in upstream LLDB that prevents
ValueObject::GetSyntheticValue to return the synthetic value from a
synthtic frontend. To temporarily work around this problem the
Swift.Optional summary formatter implements its own printing of
children. This is not a permanent solution. I will try to fix the
upstream bug ASAP.

• SwiftRuntimeTypeVisitor does currently not visit the children of
  Objective-C types; this also needs to get fixed, and is surfaced by
  the new synthtic child providers in 1 test.

Implementation Notes:

This removes a lot of enum-handling code from SwiftLanguageRuntime;
simplifying the type visitor and dynamic type resolution, adn moves it
into the Enum/Optional synthetic frontends.

[augusto2112]

Very high level first pass (didn't look too deeply into the code yet).

[adrian-prantl]

@swift-ci test

[adrian-prantl]

@swift-ci test

[JDevlieghere]

🥳