parse.y: reduce duplicate code

        parse.y: fix interpolated string literal dedent

        * parse.y (heredoc_dedent): fix interpolated string literal dedent,
          remove indentations from only nodes with the newline flag.
          [ruby-core:85983] [Bug #14584]

        * parse.y (here_document): set the newline flag on literal string
          nodes starting at the beginning of line.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62909 b2dd03c8-39d4-4d8f-98ff-823fe69b080e