[ConstraintSystem] Improve connected components printing in the type inference algorithm debug output

[amritpan]

The connected components will explicitly state the type variables currently in scope, their initial possible bindings, and the next type variable eligible for evaluation. Additionally, any new type variables, constraints or equivalence classes found, added, and removed as a result of the solver step attempt will be listed under a new section called Changes

For each component under evaluation, an Current Bindings section will present the initial type variable bindings and disjunctions/conjunctions.

For example:

---Connected components---
  0: $T1 $T2 $T3 $T4
  1: $T5
    (solving component #1
      ($T5 bindings={(subtypes of) Int})
      Initial bindings: $T5 := Int
      (attempting type variable $T5 := Int
       ...

will now print as:

---Connected components---
  0: $T1 $T2 $T3 $T4
  1: $T5
    (solving component #1
      Type variables in scope = [$T5]
      (Current Binding: 
        ($T5 [with possible bindings: (subtypes of) Int])
      )
      (attempting type variable $T5 := Int
       ...

When a new type variable, equivalence class or constraint is removed or added during a solving attempt, a Changes section will present these updated type variables and constraints. For example:

 (attempting disjunction choice $T1 bound to decl Swift.(file).AdditiveArithmetic.+ : <Self where Self : AdditiveArithmetic> (Self.Type) -> (Self, Self) -> Self [[locator@0x13e0a0b98 [OverloadedDeclRef@/...]]];
          ...
          ($T2 [with possible bindings: (subtypes of) Int])
          ($T3 [attributes: [literal: string]] [with possible bindings: (subtypes of) Int])
          Initial bindings: $T2 := Int
          (attempting type variable $T2 := Int
         ... 

will now print the following Changes:

  (attempting disjunction choice $T1 bound to decl Swift.(file).AdditiveArithmetic.+ : <Self where Self : AdditiveArithmetic> (Self.Type) -> (Self, Self) -> Self [[locator@0x13b903798 [OverloadedDeclRef@/...]]];
          ...
          (Changes:
            (New Bound Type: 
              > $T1 := ($T7, $T7) -> $T7
              > $T7 := Int
            )
            (New Type Variable: 
              > $T7
            )
            (Added Constraint: 
              > $T2 transitive conformance to AdditiveArithmetic [[locator@0x13b933158 [OverloadedDeclRef@/... -> opened generic -> type parameter requirement #0 (conformance)]]];
              > $T2 operator arg conv $T7 [[locator@0x13b905e80 [Binary@/... -> apply argument -> comparing call argument #0 to parameter #0]]];
              > $T3 transitive conformance to AdditiveArithmetic [[locator@0x13b933158 [OverloadedDeclRef@/... -> opened generic -> type parameter requirement #0 (conformance)]]];
              > $T3 operator arg conv $T7 [[locator@0x13b905f20 [Binary@/... -> apply argument -> comparing call argument #1 to parameter #1]]];
            )
            (Removed Constraint: 
              > (_const $T2, _const $T3) -> $T4 applicable fn $T1 [[locator@0x13b905990 [Binary@/... -> apply function]]];
            )
          )
          (Current Bindings: 
            ($T2 [with possible bindings: (subtypes of) Int])
            ($T3 [attributes: [literal: string]] [with possible bindings: (subtypes of) Int])
          )
          (attempting type variable $T2 := Int
          ... 

[xedin]

LGTM! I left a few small comments inline.

[xedin]

This and conjunction section prints a selected disjunction/conjunction instead of all of them but let’s address that in a follow up PR instead of holding this one.

[xedin]

@swift-ci please test