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

[hborla]

Order VarDecls before other kinds of declarations 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, which enables solving code like this quickly:

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)
}

This resolves the source compatibility regression in penny-bot.

Resolves: rdar://125674969

[hborla]

@swift-ci please test source compatibility

[hborla]

@swift-ci please smoke test

[hborla]

My added test failed because I added it to a file that overloads count(where:)! The source compat failures are a UPASS of RxReactiveObjc, and importantly, penny-bot passed.

[hborla]

@swift-ci please smoke test

[xedin]

[rintaro]

@swift-ci please smoke test macOS

[rintaro]

(The failure looked unrelated lldb-api :: lang/swift/embedded/frame_variable/TestSwiftEmbeddedFrameVariable.py)
https://ci.swift.org/job/swift-PR-macos-smoke-test/12820/console

[hborla]

Same failure again

@swift-ci please smoke test macOS

[rintaro]

@swift-ci please smoke test macOS

[hborla]

@swift-ci please smoke test macOS