Fix line-ending empty interpolation

Forward port fix from 2.11.

Also fix position of interpolated parts from 2.10.

Author: som-snytt

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -742,17 +742,22 @@ object Scanners {
             if (token == INTERPOLATIONID) {
               nextRawChar()
               if (ch == '\"') {
-                nextRawChar()
-                if (ch == '\"') {
+                if (lookaheadChar() == '\"') {
+                  nextRawChar()
+                  //offset += 3   // first part is positioned at the quote
                   nextRawChar()
                   stringPart(multiLine = true)
                 }
                 else {
+                  nextChar()
                   token = STRINGLIT
                   strVal = ""
                 }
               }
-              else stringPart(multiLine = false)
+              else {
+                //offset += 1   // first part is positioned at the quote
+                stringPart(multiLine = false)
+              }
             }
             else {
               nextChar()

tests/pos/t7919.scala

@@ -0,0 +1,11 @@
+
+object X {
+  val x = s""
+  val y = true
+}
+
+/* was:
+4 |  val y = true
+  |  ^^^
+  |  end of statement expected but 'val' found
+*/