K2: Implement partially constrained lambda analysis (PCLA)

It's expected to partially mimic the behavior of what
previously was called builder inference, but with more clear contracts
(documentation is in progress, though)

See a lot of fixed issues in the later commits with test data,
especially [red-to-green]

^KT-59791 In Progress

core/descriptors/src/org/jetbrains/kotlin/types/checker/ClassicTypeSystemContext.kt

@@ -622,6 +622,11 @@ interface ClassicTypeSystemContext : TypeSystemInferenceExtensionContext, TypeSy
         errorSupportedOnlyInTypeInference()
     }
 
+    @K2Only
+    override fun createSubstitutionFromSubtypingStubTypesToTypeVariables(): TypeSubstitutorMarker {
+        error("Only for K2")
+    }
+
     override fun TypeSubstitutorMarker.safeSubstitute(type: KotlinTypeMarker): KotlinTypeMarker {
         require(type is UnwrappedType, type::errorMessage)
         require(this is TypeSubstitutor, this::errorMessage)