Surivive bad selftypes

These are sometimes detected after they are computed. Need to avoid
a crash in AndType due to an operand that is not a value type.

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3477,7 +3477,7 @@ object Types {
       if (selfTypeCache == null)
         selfTypeCache = {
           val given = cls.givenSelfType
-          if (!given.exists) appliedRef
+          if (!given.isValueType) appliedRef
           else if (cls is Module) given
           else if (ctx.erasedTypes) appliedRef
           else AndType(given, appliedRef)

tests/neg/bad-selftype.scala

@@ -0,0 +1,6 @@
+trait x0[T] { self: x0 => }           // error
+
+trait x1[T] { self: (=> String) => }  // error
+
+trait x2[T] { self: ([X] => X) => }   // error
+

tests/neg/parser-stability-17.scala

@@ -0,0 +1,2 @@
+trait x0[] {      x0: x0 => }
+        class x0[x1] extends x0[x0 x0] x2 x0