[FIR] Prevent missing diagnostics on free lambdas

The change in `FirPsiDiagnosticTestGenerated.Resolve#testCast`
only highlights the existing
problem that we don't assign a
valid type to `_`.

^KT-58906 Fixed

compiler/fir/analysis-tests/testData/resolve/cast.kt

@@ -4,4 +4,4 @@ val y = 2 as Any
 val f = fun() = 3 as Any
 val g = {}
 val h: (String) -> Boolean = { _ -> false }
-val hError = { _ -> true }
+val hError = { <!VALUE_PARAMETER_WITH_NO_TYPE_ANNOTATION!>_<!> -> true }