Fix line-ending empty interpolation

[som-snytt]

Forward port fix from 2.11.

scala/scala@48413b4

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[som-snytt]

My merge was thrown off by the absence of another commit.

Including scala/scala@a8edefc

I couldn't get neg/t5510.scala to work under vulpix because it reports an error past the last line of input; I think there is an escape hatch? but I haven't looked it up: error-nopos or similar. I wonder if the line counting problem was also fixed on Scala 2.

The error tries to be positioned on the last literal part (not sure that is optimal). That position should not be the opening quote mark, as in .22, when there is one part. (I'd expect either always caret at erroneous open quote, or at last position it expected a closer.) Correcting the position of the part aligns with Scala 2 for now.

Verifying manually; this is a 7-line file but last error is line 8:

sbt:dotty> dotc tests/neg/t5510.scala
[snip]
-- Error: tests/neg/t5510.scala:3:12 -------------------------------------------
3 |  def s = s"xxx  // error
  |            ^
  |            unclosed string literal
-- [E040] Syntax Error: tests/neg/t5510.scala:4:0 ------------------------------
4 |} // error
  |^
  |';' expected, but '}' found
-- [E080] Syntax Error: tests/neg/t5510.scala:5:0 ------------------------------
5 |trait U { // error
  |^^^^^^^^^^^^^^^^^^
  |Expected a toplevel definition
-- Error: tests/neg/t5510.scala:6:14 -------------------------------------------
6 |  def s = s"""xxx // error
  |              ^
  |              unclosed multi-line string literal
-- Error: tests/neg/t5510.scala:8:0 --------------------------------------------
8 |
  |^
  |unclosed multi-line string literal
5 errors found

[som-snytt]

Or, the position fix causes position check failures. Maybe it was removed for a reason. I'll revert that and save it for another day.