[6.1][rbi] Change Region Based Isolation for closures to not use the AST and instead just use SIL.

[gottesmm]

This changes how region based isolation emits errors on closures. Specifically, previously we would attempt to combine AST information with SIL level information by attempting to create a correspondence in between the captures of a partial_apply and the captures of the ClosureExpr attached as a SILLocation to the partial_apply. This turns out to not be sound due to cases like the following where a partial_apply has captures from multiple different closures:

class NS {}

func test(_ x: NS, _ y: NS) {
  func local() {
    print(x)
  }
  {
    _ = y
    local()
  }()
}

which codegens to:

// closure #1 in test(_:_:)
// Isolation: concurrent
sil private [ossa] @$s4testAAyyAA2NSC_ACtFyyXEfU_ : $@convention(thin) (@guaranteed NS, @guaranteed NS) -> () {
// %0 "y"                                         // users: %4, %2
// %1 "x"                                         // users: %6, %3
bb0(%0 : @closureCapture @guaranteed $NS, %1 : @closureCapture @guaranteed $NS):

the partial_apply for closure 1 will have a SILLocation with a ClosureExpr that only contains y. If one attempts to process the closure parameters backwards using that information, one will get the wrong information. Instead of doing this, I introduced a new utility function called findClosureUses that recursively searches through closures looking for a use of the capture to emit a note upon.

If one looks at the PR it looks a little large, but most of the changes are due to my needing to add a new instruction called ignored_use. This is because the AST information that we used to use allowed us to emit diagnostics on code like:

Task {
   _ = x
}

sadly at the SIL level, we did not actually emit any code for _ = x so the findClosureUse function I added did not have a SILLocation that could be used. So I introduced a new SILInstruction called ignored_use that models _ = x appropriately namely as an instruction that suppresses unused variable warnings (that is a fake use).

Scope: Added a utility function that uses SIL instead of AST information to find closure uses to emit a diagnostic upon. This works around correctness issues exposed by the old implementation. This only comes up if we were going to already emit a diagnostic but needed to find the use that caused the diagnostic to need to be emitted. By taking this, we will eliminate crashes along this code path when we attempt to diagnose. So this cannot break any code that already works and will just provide a better experience for users since the compiler will emit the correct diagnostic instead of crashing or emitting the wrong diagnostic.
Risk: Low
Testing: Added compiler tests.
Issue: rdar://142661388
Reviewer: @nate-chandler
Original PRs:
#79028
#78837

[gottesmm]

@swift-ci test

[gottesmm]

@swift-ci test

[nate-chandler]

Reviewed for CCC

[gottesmm]

@swift-ci test

[gottesmm]

We decided this is took big of a change for 6.1. I am going to do a smaller change that doesn't change approaches and fix a few of the issues people have been hitting.