RFR: 8319196: ExecutableElement.getReceiverType doesn't return receiver types for methods loaded from bytecode [v7]

[Liam Miller-Cushon]

> Hello,
> 
> Please consider this fix for [JDK-8319196: ExecutableElement.getReceiverType doesn't return receiver types for methods loaded from bytecode](https://bugs.openjdk.org/browse/JDK-8319196).
> 
> As discussed in [the accompanying CSR](https://bugs.openjdk.org/browse/JDK-8319330), `ExecutableElement.getReceiverType()` and `ExecutableType.getReceiverType()` currently returns `NoType` unless the receiver parameter syntax was present in source, which also means that `getReceiverType()` doesn't work for methods loaded from class files.
> 
> This fix updates the implementation to return the implicit receiver type of methods that do not explicitly declare a receiver parameter in source, including for methods that were loaded from class files.

Liam Miller-Cushon has updated the pull request incrementally with one additional commit since the last revision:

  Add a null check

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16482/files
  - new: https://git.openjdk.org/jdk/pull/16482/files/3172f45e..7aafb3bc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16482&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16482&range=05-06

  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16482.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16482/head:pull/16482

PR: https://git.openjdk.org/jdk/pull/16482