SI-2464 Resiliance against missing InnerClass attributes

Adapted from scalac commit 2a19cd56258884e25f26565d7b865cc2ec931b23 by
Jason Zaugg, but without the testing infrastructure added:

A classfile in the wild related to Vaadin lacked the InnerClasses
attribute. As such, our class file parser treated a nested enum
class as top-level, which led to a crash when trying to find its
linked module.

More details of the investigation are available in the JIRA comments.

The test introduces a new facility to rewrite classfiles.

This commit turns this situation into a logged warning, rather
than crashing. Code by paulp, test by yours truly.

Author: smarter

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -227,8 +227,12 @@ class ClassfileParser(
         // seal java enums
         if (isEnum) {
           val enumClass = sym.owner.linkedClass
-          if (!(enumClass is Flags.Sealed)) enumClass.setFlag(Flags.AbstractSealed)
-          enumClass.addAnnotation(Annotation.makeChild(sym))
+          if (!enumClass.exists)
+            ctx.warning(s"no linked class for java enum $sym in ${sym.owner}. A referencing class file might be missing an InnerClasses entry.")
+          else {
+            if (!(enumClass is Flags.Sealed)) enumClass.setFlag(Flags.AbstractSealed)
+            enumClass.addAnnotation(Annotation.makeChild(sym))
+          }
         }
       } finally {
         in.bp = oldbp