SI-8442 Ignore stub annotation symbols in `AnnotationInfo#matches`

And update the java `ClassFileParser` to create distinguished
`StubClassSymbol`s, rather that a regular `ClassSymbol`s, when
encountering a deficient classpath. This brings it into line
with `Unpickler`, which has done as much since a55788e275f.

This stops the enclosed test case from crashing when determining
if the absent symbol, `A_1`, is a subclass of `@deprecated`.

This is ostensibly fixes a regression, although it only worked in
`2.10.[0-3]` by a fluke: the class file parser's promiscious
exception handling caught and recovered from the NPE introduced
in SI-7439!

	% javac -d /tmp test/files/run/t8442/{A,B}_1.java &&  qbin/scalac -classpath /tmp -d /tmp test/files/run/t8442/C_2.scala && (rm /tmp/A_1.class; true) && scalac-hash v2.10.0 -classpath /tmp -d /tmp test/files/run/t8442/C_2.scala
	warning: Class A_1 not found - continuing with a stub.
	warning: Caught: java.lang.NullPointerException while parsing annotations in /tmp/B_1.class
	two warnings found

1 files changed, 7 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala b/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
index 2955986a7e..6991cfa37b 100644
--- a/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
+++ b/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
@@ -436,9 +436,13 @@ abstract class ClassfileParser {
       // SI-5593 Scaladoc's current strategy is to visit all packages in search of user code that can be documented
       // therefore, it will rummage through the classpath triggering errors whenever it encounters package objects
       // that are not in their correct place (see bug for details)
-      if (!settings.isScaladoc)
-        warning(s"Class $name not found - continuing with a stub.")
-      return NoSymbol.newClass(name.toTypeName)
+
+      // TODO More consistency with use of stub symbols in `Unpickler`
+      //   - better owner than `NoSymbol`
+      //   - remove eager warning
+      val msg = s"Class $name not found - continuing with a stub."
+      if (!settings.isScaladoc) warning(msg)
+      return NoSymbol.newStubSymbol(name.toTypeName, msg)
     }
     val completer     = new global.loaders.ClassfileLoader(file)
     var owner: Symbol = rootMirror.RootClass