Remove tail recursion and enable cancellation

[mattwar]

Fixes devdiv 1089540

The problem was a stack overflow in the formatter when pasting a file with a enormous array initializer. The culprit was a method FormattingContext.GetEndTokenForRelativeIndentationSpan that was making a recursive call after looking forward to the next token, attempting to find the end token for a relative indentation operation. Fortunately it was simply tail recursive and was easy to rewrite as a loop.

Also noticed that the whole operation when successful was taking tremendous amount of time (on order of 10 minutes), and was stuck in this recursive method for a long time without ability to cancel, so I plumbed cancellation through to its loop.

Also noticed that the dialog that pops up on slow execution of formatting did not have a cancel button, so I enabled that too.

@heejaechang @jasonmalinowski @Pilchie please review.

[heejaechang]

👍

[Pilchie]

Why did you close this?

[jasonmalinowski]

It was reopened as #3229 against the right branch.