In the reproduction in scala/bug#11679, an SBT build that uses
`--release 8` and as such does not have `sun._` on the classpath,
`APIPhase` (a custom compiler phase in SBT/Zinc that serializes the API
of each file to perform change detection in incremental compilation)
is attempting to serialize the API of the member
`@CallerSensitive ClassLoader getParent` that
`de.sciss.synth.proc.impl.MemoryClassLoader` inherits from
`j.l.ClassLoader`.

Until that point, scalac was doing okay without having a classfile
for `CallerSensitve` -- it just used a `StubSymbol` in its place.
Scala's pickle phase only serializes `.decls`, not `.members`.

When `APIPhase` filters the list of annotations:

  https://github.com/sbt/zinc/blob/4b414b6677/internal/compiler-bridge/src/main/scala/xsbt/ExtractAPI.scala#L789-L800

for those that extend `scala.StaticAnnotation`, the stub symbol fails
and aborts compilation. Static annotations are part of the API because
the are could affect how client code is compiled.

This commit changes `AnnotationInfo.isStatic` to return false
for annotations to absent classfiles. It also makes sure that we
still have a hard-error if annotations based on absent classfiles
are processed by the pickle phase.

I have manually tested this commit with the SBT project in the bug.

Fixes scala/bug#11679