Fix #3909: Backslash to break line is inconsistent in 'for ... of' and 'for ... in' loops

[zdenko]

Fixes #3909.
A syntax error is thrown when the backslash is used in for of and for in loops, and in or of tags aren't in the same line with for tag.
The issue is in Lexer::lineToken where for tag is closed (@seenFor = no). This PR corrects this by checking the previous tag, e.g. tag before the TERMINATOR.

x = ( key for \
  own key of obj )

x = ( key for own \
  key of obj )

x = ( key for own key \
  of obj )

x = ( key for \
  key of obj )

x = ( key for \
  key of obj )

x = ( key for key \
  of obj )

x = ( key for \
  key in obj )

x = ( key for \
  key in obj )

x = ( key for key \
  in obj )