[ConstraintSystem] Print constraint simplification in type inference debug output

[amritpan]

The constraint currently being evaluated for the overload, any new constraints added as a result of this evaluation (or removed), and the result of the simplification will be printed out to show the constraint simplification process of the constraint solver.

The following is a constraint simplification attempt for an overload choice of $T0 := (String, String) -> String). The initial constraint is simplified and two new smaller constraints are generated to evaluate the expression for this overload. The initial constraint is then removed and the simplification is shown as simplified:

(considering -> (_const $T1, _const $T2) -> $T3 applicable fn $T0 [[locator@0x13704a3f0 [Binary@/... -> apply function]]];
  (simplification result: 
    (added Constraint: $T1 operator arg conv String [[locator@0x13704a4c0 [Binary@/... -> apply argument -> comparing call argument #0 to parameter #0]]];)
    (added Constraint: $T2 operator arg conv String [[locator@0x13704a560 [Binary@/... -> apply argument -> comparing call argument #1 to parameter #1]]];)
    (removed Constraint: (_const $T1, _const $T2) -> $T3 applicable fn $T0 [[locator@0x13704a3f0 [Binary@/... -> apply function]]];)
  )
  (outcome: simplified)
)

If the constraint simplification attempt fails, it will print the constraints followed by an error result:

(considering -> (_const $T2, _const $T3) -> $T4 applicable fn $T1 [[locator@0x13f09b190 [Binary@/... -> apply function]]];
  (simplification result:
    (added constraint: $T2 operator arg conv String [[locator@0x13f09b680 [Binary@/...-> apply argument -> comparing call argument #0 to parameter #0]]];)
    (added constraint: $T3 operator arg conv String [[locator@0x13f09b720 [Binary@/... -> apply argument -> comparing call argument #1 to parameter #1]]];)
    (removed constraint: (_const $T2, _const $T3) -> $T4 applicable fn $T1 [[locator@0x13f09b190 [Binary@/... -> apply function]]];)
    (failed constraint (_const $T2, _const $T3) -> $T4 applicable fn $T1 [[locator@0x13f09b190 [Binary@/... -> apply function]]];)
  )
  (outcome: error)
)

If an added constraint is a disjunction kind, its choices will be nested:

(considering -> @lvalue Int? operator arg conv $T5 [[locator@0x13e00d360 [Binary@/Users/amritpankaur/test.swift:72:18 -> apply argument -> comparing call argument #1 to parameter #1 -> @autoclosure result]]];
  (simplification result:
    (added constraint: disjunction [[locator@0x13e00d360 [Binary@/Users/amritpankaur/test.swift:72:18 -> apply argument -> comparing call argument #1 to parameter #1 -> @autoclosure result]]]:
    >  [favored]  Int? bind Int?? [deep equality] [[locator@0x13e00d360 [Binary@/Users/amritpankaur/test.swift:72:18 -> apply argument -> comparing call argument #1 to parameter #1 -> @autoclosure result]]];
    >             Int? operator arg conv Int?? [optional-to-optional] [[locator@0x13e00d360 [Binary@/Users/amritpankaur/test.swift:72:18 -> apply argument -> comparing call argument #1 to parameter #1 -> @autoclosure result]]];
    >             Int? operator arg conv Int?? [value-to-optional] [[locator@0x13e00d360 [Binary@/Users/amritpankaur/test.swift:72:18 -> apply argument -> comparing call argument #1 to parameter #1 -> @autoclosure result]]];)
  (removed constraint: @lvalue Int? operator arg conv $T5 [[locator@0x13e00d360 [Binary@/Users/amritpankaur/test.swift:72:18 -> apply argument -> comparing call argument #1 to parameter #1 -> @autoclosure result]]];)
  )
  (outcome: simplified)
)

This adds to the solver step Changes printed in #59971.

[xedin]

Looks great, I left a couple of minor comments inline.

[xedin]

@swift-ci please test

[xedin]

@swift-ci please test macOS platform