[6.0][ConstraintSystem] Order VarDecls before other kinds of decls in disjunctions.

[hborla]

• Explanation: The implementation of count(where:) caused a source compatibility failure in penny-bot. The following code example was reduced from the project:

  func f(
    a: String?,
    b: String?,
    c: Int,
    d: String?,
    e: String?
  ) -> Int {
    return (a?.unicodeScalars.count ?? 0) +
    (b?.unicodeScalars.count ?? 0) +
    c +
    (d?.unicodeScalars.count ?? 0) +
    (e?.unicodeScalars.count ?? 0)
  }

  To fix this, order VarDecls before other kinds of declarations in a disjunction because they are effectively favored over functions when the two are in the same overload set. This disjunction order allows SK_UnappliedFunction to prune later overload choices that are functions when a solution has already been found with a property.

• Scope: Only impacts overload sets that contain both VarDecls and other kinds of declarations.

• Risk: Changing type variable binding order always carries some amount of risk for code that happened to find a solution faster with the old binding order, but it's rare to have functions and variables in the same overload set, so I would call this a relatively low risk constraint system change.

• Testing: Added a new type checker performance test. Source compatibility testing for penny-bot also succeeded on main.

• Reviewer: @xedin

• Main branch PR: [ConstraintSystem] Order VarDecls before other kinds of decls in disjunctions. #72755

[hborla]

@swift-ci please test