Add --include-trivia flag to swift-parser-cli.

[allevato]

This is something that I've wanted to have during development, especially when manipulating trivia in swift-format.

---

Manually tested example:

$ swift-parser-cli print-tree --include-trivia <<'EOF'
heredoc> /*xyz*/ func foo() {  // blah
heredoc>   }
heredoc> EOF

SourceFileSyntax children=2
  0: CodeBlockItemListSyntax children=1
    0: CodeBlockItemSyntax children=1
      0: FunctionDeclSyntax children=4
        0: funcKeyword leadingTrivia=[blockComment("/*xyz*/"), spaces(1)] trailingTrivia=spaces(1)
        1: identifier("foo")
        2: FunctionSignatureSyntax children=1
          0: ParameterClauseSyntax children=3
            0: leftParen
            1: FunctionParameterListSyntax
            2: rightParen trailingTrivia=spaces(1)
        3: CodeBlockSyntax children=3
          0: leftBrace trailingTrivia=[spaces(2), lineComment("// blah")]
          1: CodeBlockItemListSyntax
          2: rightBrace leadingTrivia=[newlines(1), spaces(2)]
  1: eof leadingTrivia=newlines(1)

[allevato]

@swift-ci please test