RFR: 8255273: jshell crashes with UnsupportedOperationException: Should not get here.

[Jan Lahoda]

Consider JShell interaction like:

jshell> import java.time.*;

jshell> new Instant<tab>

This will crash the JShell with an `UnsupportedOperationException`. The ultimately reason is that:
-completions are computed, and are `InstantiationError(` and `InstantiationException(`, with a common prefix `InstantiationE`
-documentation is computed, and is for java.time.Instant

The completion state machine then gets into a state where it fills in the common prefix, and then it would like say something along the lines of "Press tab once more to see signature/documentation.", but that does not make much sense (as the documentation is computed for a wrong input - the new input is `new InstantiationE`, not `Instant`, so the documentation is already invalid for that.

(The `UnsupportedOperationException` is in place where the "Press tab once more to see signature/documentation." should happen, because the code should not get into a state like this.)

The proposed solution is to cancel the documentation if a prefix is (to be) filled.

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Commit messages:
 - Fixing compilation error.
 - Merging master into JDK-8255273
 - 8255273: jshell crashes with UnsupportedOperationException: Should not get here.

Changes: https://git.openjdk.java.net/jdk/pull/3220/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3220&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255273
  Stats: 101 lines in 2 files changed: 60 ins; 27 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3220.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3220/head:pull/3220

PR: https://git.openjdk.java.net/jdk/pull/3220