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.
merge revision(s) 62723,62724: [Backport #14584]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62909 b2dd03c8-39d4-4d8f-98ff-823fe69b080e