[4.0][Exclusivity] Don't suggest copying to a local when swapAt() is appro…

[devincoughlin]

…priate

Update static exclusivity diagnostics to not suggest copying to a local
when a call to swapAt() should be used instead. We already emit a FixIt in
this case, so don't suggest an an helpful fix in the diagnostic.

rdar://problem/32296784

[devincoughlin]

CCC Information:

Explanation: This changes updates the static diagnostic emitted when a call to swap() violates exclusive access. If the call to swap can be replaced with a call to MutableCollection.swapAt() the diagnostic text now recommends that rather than copying to a local. We already have a FixIt for this; now the main diagnostic text is consistent with the FixIt.

Scope: This is a diagnostic-only change. It is not a source-breaking change. It improves a fairly common case of an uncommon diagnostic.

Radar: rdar://problem/32296784
Risk: Low. It only affects paths on which a diagnostic would already be emitted.
Testing: I've added regression tests and run on the source compatibility suite.

[devincoughlin]

@atrick Would you mind reviewing for CCC?

[devincoughlin]

@swift-ci Please smoke test

[atrick]

🏅 Looks good for CCC.
Nice work.

[devincoughlin]

Approved by @AnnaZaks.

[devincoughlin]

@swift-ci Please smoke test and merge.

[devincoughlin]

@swift-ci Please test and merge