Don't apply "Convert to Trailing Closure" to call with multiple trailing closures

[hamishknight]

The hasTrailingClosure method has a super confusing name now, as it doesn't include multiple trailing closures. This is getting fixed in #37435, but split out this change to keep that patch as NFC as possible.

Previously we'd perform an invalid transform or hit an assertion failure when attempting to perform the "Convert to Trailing Closure" refactoring action on a call with multiple trailing closures. For now, disallow the transform for a call that already uses trailing closure syntax.

rdar://81106400

[hamishknight]

@swift-ci please test

[rintaro]

The implementation looks good. But out of curiosity, why hasTrailingClosure() is not true for multiple trailing closure call?

[hamishknight]

@rintaro It was written before multiple trailing closures got added, and I guess it never got updated to reflect that (a bunch of callers do use it to really check for the single closure case tho). With #37435, I'm separating it out into hasSingleTrailingClosure() & hasAnyTrailingClosures().