Add separate FirJava elements (class, method, parameter, type reference)

Related to KT-29218

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirMemberFunctionImpl.kt

@@ -20,7 +20,7 @@ import org.jetbrains.kotlin.fir.types.FirTypeRef
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 import org.jetbrains.kotlin.name.Name
 
-class FirMemberFunctionImpl : FirAbstractCallableMember, FirNamedFunction, FirModifiableFunction {
+open class FirMemberFunctionImpl : FirAbstractCallableMember, FirNamedFunction, FirModifiableFunction {
 
     constructor(
         session: FirSession,

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirValueParameterImpl.kt

@@ -15,7 +15,7 @@ import org.jetbrains.kotlin.fir.types.FirTypeRef
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 
-class FirValueParameterImpl(
+open class FirValueParameterImpl(
     session: FirSession,
     psi: PsiElement?,
     name: Name,

compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirUserTypeRefImpl.kt

@@ -14,11 +14,11 @@ import org.jetbrains.kotlin.fir.types.FirTypeProjection
 import org.jetbrains.kotlin.fir.types.FirUserTypeRef
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 
-class FirUserTypeRefImpl(
+open class FirUserTypeRefImpl(
     session: FirSession,
     psi: PsiElement?,
-    isNullable: Boolean
-) : FirAbstractAnnotatedTypeRef(session, psi, isNullable), FirUserTypeRef {
+    isMarkedNullable: Boolean
+) : FirAbstractAnnotatedTypeRef(session, psi, isMarkedNullable), FirUserTypeRef {
     override val qualifier = mutableListOf<FirQualifierPart>()
 
     override fun <D> transformChildren(transformer: FirTransformer<D>, data: D): FirElement {