[11u] RFR (S) 8209055: c.s.t.javac.code.DeferredCompletionFailureHandler seems to use WeakHashMap incorrectly

[Aleksey Shipilev]

Please review this backport to 11u.

Original bug:
  https://bugs.openjdk.java.net/browse/JDK-8209055

Original fix:
  http://hg.openjdk.java.net/jdk/jdk/rev/1a6aeca4a8c9

The patch does apply cleanly to 11u, but test needs fixups to compile and run. I needed to add this
line to @modules:
 + *   jdk.compiler/com.sun.tools.javac.main:+open

Testing: langtools:tier1, regression test (fails without the patch, passes with),

Thanks,
-Aleksey