RFR: [type-classes] Witness symbols with wrong owner lead to verifier errors

[Maurizio Cimadamore]

A witness lookup of the kind `T.__witness` is translated as a condy instruction.
Condy-backed `ldc` are modelled in the AST as references to `DynamicVariableSymbol`.
Such symbols have a owner. While the owner of a dynamic symbol is relatively unimportant for code generation issues (we don't need it to emit the correct bytecode), other compilation steps (such as `LambdaToMethod`) might be tricked into thinking that the dynamic symbol is actually a reference to an enclosing class symbol. This happens if the dynamic symbol owner is set to be a class symbol, instead of a method symbol.

The fix is therefore to translate the dynamic symbol of a witness lookup as a "local variable" -- that is, a variable symbol whose owner is a method (not a class) symbol.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/2021/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2021&range=00
  Stats: 70 lines in 2 files changed: 69 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/2021.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2021/head:pull/2021

PR: https://git.openjdk.org/valhalla/pull/2021