Avoid infinite loop in parser

Parser could get into an infinite loop because `acceptStatSepUnlessAtEnd`
did not make progress. I believe the concrete example was a string interpolator
where we encounter a `{ ... )`.

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

@@ -284,7 +284,16 @@ object Parsers {
     }
 
     def acceptStatSepUnlessAtEnd(altEnd: Token = EOF) =
-      if (!isStatSeqEnd && in.token != altEnd) acceptStatSep()
+      if (!isStatSeqEnd)
+        in.token match {
+          case EOF =>
+          case `altEnd` =>
+          case NEWLINE | NEWLINES => in.nextToken()
+          case SEMI => in.nextToken()
+          case _ =>
+            in.nextToken() // needed to ensure progress; otherwise we might cycle forever
+            accept(SEMI)
+        }
 
     def errorTermTree    = atPos(in.offset) { Literal(Constant(null)) }
 

tests/neg/i1779.scala

@@ -8,6 +8,6 @@ object Test {
   def f = {
     val _parent = 3
     q"val hello = $_parent"
-    q"class $_" // error // error
-  }
+    q"class $_" // error
+  } // error
 }