Kotlin: converge explicit _ setter parameter name onto K2

An explicit property setter written `set(_)` produces a value parameter
whose source spelling is the Kotlin discard placeholder `_`. The two
frontends model this differently:

  * K2 preserves the source name and exposes the parameter as `_`.
  * K1 additionally flags the parameter with
    `IrDeclarationOrigin.UNDERSCORE_PARAMETER`, which the extractor treats
    as a synthesized name (no name is written and QL synthesizes `p0`).

This made `test-kotlin1` report `p0` while `test-kotlin2` reported `_`
for the same source (library-tests/underscore-parameters).

The `UNDERSCORE_PARAMETER` origin is *also* set by K1 (and, for the
synthesized cases, by K2) on discarded parameters that do not retain a
`_` source spelling, most importantly:

  * lambda placeholder parameters, e.g. `{ index, _ -> ... }`
  * the synthesized `invoke` parameters of function types
    (funcExprs.kt lines 27/30/31/90/94)

For those, the two frontends assign *different* internal names
(`<anonymous parameter N>` under K1 vs `<unused var>` under K2), so the
synthetic-name treatment is exactly what keeps them converged at `pN`.
Removing the origin check wholesale therefore replaces one small
divergence with a much larger one.

To converge only the case both frontends agree on, we suppress the
synthetic-name treatment only when the parameter's own name is exactly
`_` (the source discard spelling that both K1 and K2 preserve). This
makes K1 emit `_` for `set(_)` (matching K2) while leaving every other
discarded parameter synthetic in both suites.

Scope of the change (verified by a full dual-suite `--learn` with
database-consistency checks, all 3333 tests passing):

  * test-kotlin1 library-tests/underscore-parameters: `p0` -> `_`
    (now identical to test-kotlin2).
  * test-kotlin2: byte-for-byte unchanged.
  * No other expected file changes (lambda / function-type discard
    parameters remain `pN` in both suites).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

java/ql/test-kotlin1/library-tests/underscore-parameters/test.expected

@@ -1 +1 @@
-| test.kt:5:9:5:9 | p0 | p0 |
+| test.kt:5:9:5:9 | _ | _ |