SI-7919 Newline after empty string interp

Consume the newline non-raw for safe handling after
single-line interpolation.

Author: som-snytt

src/compiler/scala/tools/nsc/ast/parser/Scanners.scala

@@ -475,14 +475,17 @@ trait Scanners extends ScannersCommon {
             if (token == INTERPOLATIONID) {
               nextRawChar()
               if (ch == '\"') {
-                nextRawChar()
-                if (ch == '\"') {
+                val lookahead = lookaheadReader
+                lookahead.nextChar()
+                if (lookahead.ch == '\"') {
+                  nextRawChar()                        // now eat it
                   offset += 3
                   nextRawChar()
                   getStringPart(multiLine = true)
                   sepRegions = STRINGPART :: sepRegions // indicate string part
                   sepRegions = STRINGLIT :: sepRegions // once more to indicate multi line string part
                 } else {
+                  nextChar()
                   token = STRINGLIT
                   strVal = ""
                 }

test/files/pos/t7919.scala

@@ -0,0 +1,6 @@
+
+object X {
+  val x = s""
+  val y = true
+}
+