[code-reflection] RFR: 8373571: Code model of JavaOp.ForOp does not handle (un)boxing correctly

[Maurizio Cimadamore]

This PR addresses an issue in for-each loop model generation.
The for each op factory takes three bodies:
1. the for each expression (e.g. either a collection, or an array)
2. the for each initializer (the logic that receives an element from the expression, and turns it into the expected variable type in the source code)
3. the for each body

The translation for (2) seems to contain a bug: javac erroneously assumes that the initializer body has type `(V)->V`, where `V` is the type of the induction variable declared in the source code.

This PR fixes that so that (2) has type `(ELEM)->V`, where `ELEM` is the static type of an element of the for-each expression, and `V` is the static type of the induction variable.

This translation allows uboxing operations to appear manifest in the generated model.

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Commit messages:
 - Add tests
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/750/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=750&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373571
  Stats: 79 lines in 2 files changed: 76 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/babylon/pull/750.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/750/head:pull/750

PR: https://git.openjdk.org/babylon/pull/750