[SwiftParser] Support function body and type member skipping

[rintaro]

• Introduce ParsingOptions option-set type with bodySkipping member, and add options: ParsingOptions to all Parser.parse(...) functions
• Skipped body is represented as a newly introduced SkippedDeclSyntax
  • SkippedDeclSyntax contains only one .unknown token that covers the whole skipped characters including trivia.
• Function body can be skipped as long as:
  • it does not contain nested type decls
• Type member block can be skipped as long as:
  • it does not contain nested class decls
  • it does not contain operator functions
  • it does not contain #if directives