RFR: 8340721: Clarify special case handling of unboxedType and getWildcardType [v2]

[Joe Darcy]

> First piece of a larger set of updates that may be made for JDK-8055219 to clarify the exceptional behavior of `javax.lang.model.util.Types`.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Respond to review feedback.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21193/files
  - new: https://git.openjdk.org/jdk/pull/21193/files/ff630ef9..5d7309ba

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21193&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21193&range=00-01

  Stats: 11 lines in 2 files changed: 3 ins; 3 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/21193.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21193/head:pull/21193

PR: https://git.openjdk.org/jdk/pull/21193