Add TriviaPiece.commentValue convenience property

[William-Laverty]

Summary

Adds a commentValue computed property on TriviaPiece that returns the text of comment pieces and nil for non-comment pieces.

extension TriviaPiece {
  public var commentValue: String? { get }
}

Behavior

Piece	commentValue
.lineComment("// hello")	"// hello"
.blockComment("/* world */")	"/* world */"
.docLineComment("/// docs")	"/// docs"
.docBlockComment("/** docs */")	"/** docs */"
.spaces(4), .newlines(1), etc.	nil

This complements the existing isComment property and the higher-level Trivia.docCommentValue, providing a simple way to extract the raw text from an individual trivia piece without pattern matching.

Tests

Added TriviaPieceCommentValueTests covering all four comment kinds and all non-comment piece types.

Resolves #1890

[ahoppen]

The key functionality of this property should be to strip the // etc introducers and just give the actual comment text, similar to docCommentValue, as explained in the discussion of #1890.

[William-Laverty]

Updated commentValue to strip the //, ///, /* */, and /** */ introducers and return only the actual comment content — consistent with docCommentValue behaviour as described in #1890. Tests updated accordingly. Ready for re-review!

[rintaro]

Sorry for not reviewing sooner. Could you handle cases like:

/*
 * Hello, folks!
 * // Apples and oranges
 * I am a banana!
 */

as described in #1890 (comment)
Apparently the consensus was returning "Hello, folks!\n// Apples and oranges\nI am a banana!" for this case

[rintaro]

Also adding a Release Notes entry would be appreciated 🙏
https://github.com/swiftlang/swift-syntax/blob/main/Release%20Notes/604.md

[William-Laverty]

Thanks for the review @rintaro! I've pushed a fix that:

• Handles multiline block comments with * decoration — strips leading whitespace + * + optional space from each line, and skips empty first/last lines
• Added a Release Notes entry in 604.md
• Added tests covering the multiline stripping (with and without * decoration, and doc block comments)

For the example you mentioned:

/*
 * Hello, folks!
 * // Apples and oranges
 * I am a banana!
 */

This now returns "Hello, folks!\n// Apples and oranges\nI am a banana!" as expected.