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

[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).

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Commit messages:
 - JDK-8300857: State return value for Types.asElement(NoType) explicitly

Changes: https://git.openjdk.org/jdk/pull/12159/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12159&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300857
  Stats: 13 lines in 1 file changed: 13 ins; 0 del; 0 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