[Diagnostics] fix trivia merging and transfer

[AppAppWorks]

This PR will deprecate the incorrectly implemented methods Trivia.merging(_:) and Trivia.merging(triviaOf:) which aren't behaving in the way as advertised by their documentation.

The unexpected behaviors of Trivia.merging(_:) are causing subtle issues in trivia transfers. For example, applying the Fix-it that moves a misplaced throw clause from after the arrow to before the arrow will result in an extraneous whitespace before the return type.

func test() -> throws(any Error) Int 
// after Fix-it
func test() throws(any Error) ->  Int 
// expected
func test() throws(any Error) -> Int 

Even worse, sometimes the trivia of the moved node will be destroyed,

func test() -> throws(any Error)/**/ Int 
// after Fix-it
func test() throws(any Error) ->  Int 
// expected
func test() throws(any Error) -> /**/ Int 

Making use of the newly introduced methodsTrivia.mergingCommonPrefix and Trivia.mergingCommonSuffix, this PR will also fix these issues.

[ahoppen]

Could you add an entry in Release Notes/601.md for the public API changes?

[ahoppen]

What do you think of continuing to have a single merging function that merges both a common prefix and a common suffix? I would expect that that’s what most callers would prefer. I doubt that anyone is actively thinking about whether they want to merge the prefix or the suffix?

[AppAppWorks]

The issue here is merging doesn't actually merge by common prefix or common suffix despite its documentation, changing its behavior may break client code that depends on it, that's why I suggested to deprecate it.

[ahoppen]

I would consider that a bug fix and thus an acceptable behavior change.