RFR: JDK-8300857: State return value for Types.asElement(NoType) explicitly [v2]

[Joe Darcy]

> Just sending out the proposed API changes for now, will add tests later.
> 
> Please also review the corresponding CSR [JDK-8300951](https://bugs.openjdk.org/browse/JDK-8300951).

Joe Darcy has updated the pull request incrementally with two additional commits since the last revision:

 - Applease jcheck.
 - Refine spec, add regression test.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12159/files
  - new: https://git.openjdk.org/jdk/pull/12159/files/2defd826..a60301ce

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12159&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12159&range=00-01

  Stats: 109 lines in 3 files changed: 103 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12159.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12159/head:pull/12159

PR: https://git.openjdk.org/jdk/pull/12159