Add buildFinalResult to CodeBlockItemListBuilder to ensure newline between expressions

[mateusrodriguesxyz]

This ensures that the following usage of CodeBlockItemListBuilder produces a valid block where expressions are separated by newlines:

CodeBlockItemListSyntax {
    "let a = 1"
    "let b = 2"
    "let c = 3"
}

Something similar it's already done for comma separated lists here.

I've also modified the collapse function to avoid adding unnecessary separators to a macro expansion. This change is important because if someone uses CodeBlockItemListBuilder as in testDontAddIndentationWhenCollapsingBody, they might be surprised by unexpected empty lines if collapse adds separator unconditionally.

func f() {
#sourceLocation(file: "test.swift", line: 3)

        let x: Int = 1

#sourceLocation()
}

[ahoppen]

Thank you @mateusrodriguesxyz. Could you add a test case for this?

[mateusrodriguesxyz]

@ahoppen done!

[ahoppen]

Just one nitpick, otherwise looks good to me.

[ahoppen]

@swift-ci Please test

[ahoppen]

Not entirely sure why but there seems to be a build failure on Linux only. 🤔 Could you take a look at it @mateusrodriguesxyz?

[mateusrodriguesxyz]

Not entirely sure why but there seems to be a build failure on Linux only.

It's because I used SE-0380 which it's not available in Swift 5.8 😅 It's fixed now. Could you test again please? @ahoppen

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test