[SILGen] Fix leak in thunk for async throwing swift conformance to ObjC requirement with optional completion.

[nate-chandler]

Fix a leak when emitting the native to foreign thunk for an async function which fulfills an Objective-C protocol requirement which can be fulfilled with either a value or an error via a nullable completion.

Previously, the SIL in question used to look like this:

  %maybe_completion = ...
  try_apply %impl..., normal success, ...

success(%value):
  switch_enum %maybe_completion...
      case some!enumelt: invoke
      case none!enumelt: ignore

ignore:
  br join

invoke(%completion):
  %some_value = enum Optional, some!enumelt, %value // consumes %value
  %guaranteed_some_value = begin_borrow %some_value
  %none_error = enum Optional, none!enumelt
  apply %completion(%guaranteed_some_value, %none_error)
  end_borrow %guaranteed_some_value
  destroy_value %some_value
  br join

join:
  destroy_value %maybe_completion
  ...

which leaks %value on the codepath through ignore.

Note that %value is consumed by the enum instruction, but %completion is invoked with %guaranteed_some_value, a guaranteed value. So there is no need to consume %value in invoke.

Here, %value itself is borrowed and forwarded into an enum instruction whose result is passed to %completion:

  %maybe_completion = ...
  try_apply %impl..., normal success, ...

success(%value):
  switch_enum %maybe_completion...
      case some!enumelt: invoke
      case none!enumelt: ignore

ignore:
  br join

invoke(%completion):
  %guaranteed_value = begin_borrow %value
  %guaranteed_some_value = enum Optional, some!enumelt, %guaranteed_value
  %none_error = enum Optional, none!enumelt
  apply %completion(%guaranteed_some_value, %none_error)
  end_borrow %guaranteed_some_value
  br join

join:
  destroy_value %maybe_completion
  destroy_value %value
  ...

Because an argument scope was already being created and a cleanup was already being pushed for %value, nothing more is required to fix the issue than to reorder the enum and the borrow.

rdar://119732084

[nate-chandler]

@swift-ci please test

[nate-chandler]

@swift-ci please test linux platform

[nate-chandler]

@swift-ci please test windows platform

[nate-chandler]

@swift-ci please test