RFR: 8293897: Synthetic final modifier is part of the AST for a try-with-resource resource

[Jan Lahoda]

The resources in try-with-resource are implicitly final (unless they are marked as final in the source code). But, in javac, the `FINAL` modifier is part of the modifiers on the AST, which is in contrary to the usual direction of having the AST closely model the source code.

The proposal here is to not add the synthetic `FINAL` modifier to the AST, but add it to the given `VarSymbol`. The positions of the `ModifiersTree` are also tweaked, so that for a synthetic tree, the positions are `NOPOS`.

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Commit messages:
 - 8293897: Final modifier auto inserted as part of try with resources statement

Changes: https://git.openjdk.org/jdk/pull/10348/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10348&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293897
  Stats: 71 lines in 3 files changed: 67 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10348.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10348/head:pull/10348

PR: https://git.openjdk.org/jdk/pull/10348