Allow infix operators at start of line

[odersky]

Examples:

val x = 1
  + 2
  + 3

val condition = 
      x > 0
  ||  xs.exists(_ > 0)
  ||  xs.isEmpty

Based on #7024.

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[odersky]

The Dotty compiler gives a warning under -language:Scala2 if the interpretation of a leading infix operator changes. E.g for

  c ! "hello"
    ! "world"

you get:

14 |    ! "world"
   |    ^
   |Line starts with an operator;
   |it is now treated as a continuation of the expression on the previous line,
   |not as a separate statement.

It would be good if Scala 2.14 could make the same situation an error. Then it would be easy to upgrade to the new meaning, without risk of breaking changes.

[odersky]

This PR is now broken out from #7024, since it is independent, and apparently less controversial.

[liufengyun]

Otherwise, LGTM

[liufengyun]

If we remove the field allowLeadingInfixOperators, all the tests pass. However, the field makes a semantic difference for the following program:

@main def Test = {
  val a = 5
  val x = 1
    + //
    `a` * 6

  assert(x == 1)
}

Maybe add the code above as a run test.