RFR: 8372635: Lambdas do not copy over SYNTHETIC flag for local variables

[Jan Lahoda]

Consider case like:

Object o = ...;

//synthetic variable for the instanceof's expression
boolean b = o.toString() instanceof String str && str.isEmpty();

Runnable r = () -> {
        //synthetic variable for the instanceof's expression
        boolean b = o.toString() instanceof String str && str.isEmpty();
};


There are synthetic variables generated for `o.toString()`, to ensure the method is invoked only once for each `instanceof`. These variables are marked as synthetic in `TransPatterns` (and then consequently omitted in `LocalVariableTable`), but when the flag is lost for the variable in the lambda.

This PR proposes to keep the flag, as suggested in the bug.

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

Commit messages:
 - 8372635: Lambdas do not copy over SYNTHETIC flag for local variables

Changes: https://git.openjdk.org/jdk/pull/28724/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28724&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372635
  Stats: 139 lines in 2 files changed: 138 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28724.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28724/head:pull/28724

PR: https://git.openjdk.org/jdk/pull/28724