[jdk18] RFR: 8278930: javac tries to compile a file twice via PackageElement.getEnclosedElements

[Jan Lahoda]

When using the Elements API, it may happen that an inner class is being completed before its enclosing class. The code is generally prepared for that, and will complete the enclosing class before filling in the content of the inner class.

An issue happens if the enclosing class is completed from source. Then, since [JDK-8224922](https://bugs.openjdk.java.net/browse/JDK-8224922), the innerclass' `ClassSymbol.classfile` will be set to the source file as well, and then javac will try to fill the innerclass from the source code again, leading to duplicate class error. (Before JDK-8224922, the innerclass was, as far as I can tell, completed from the classfile, which didn't lead to this specific problem, but is unlikely to be completely correct either.)

The proposal in this fix is to detect the situation where the nested class was already filled after the enclosing class has been completed, and skip further processing.

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Commit messages:
 - 8278930: javac tries to compile a file twice via PackageElement.getEnclosedElements

Changes: https://git.openjdk.java.net/jdk18/pull/85/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk18&pr=85&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278930
  Stats: 176 lines in 2 files changed: 174 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk18/pull/85.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk18 pull/85/head:pull/85

PR: https://git.openjdk.java.net/jdk18/pull/85