FIR checker: Separate FirExternalDeclarationChecker as it is

JVM-specific.

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/checkers/CheckersContainers.kt

@@ -16,3 +16,8 @@ fun FirSessionFactory.FirSessionConfigurator.registerExtendedCommonCheckers() {
     useCheckers(ExtendedExpressionCheckers)
     useCheckers(ExtendedDeclarationCheckers)
 }
+
+// TODO: Move this to different, JVM-specific module?
+fun FirSessionFactory.FirSessionConfigurator.registerJvmCheckers() {
+    useCheckers(JvmDeclarationCheckers)
+}

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/checkers/CommonDeclarationCheckers.kt

@@ -18,7 +18,6 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
         FirModifierChecker,
         FirConflictsChecker,
         FirConflictingProjectionChecker,
-        FirExternalDeclarationChecker,
         FirConstDeclarationChecker,
     )
 

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/checkers/JvmDeclarationCheckers.kt

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.checkers
+
+import org.jetbrains.kotlin.fir.analysis.checkers.declaration.*
+import org.jetbrains.kotlin.fir.analysis.checkers.declaration.jvm.FirJvmExternalDeclarationChecker
+
+// TODO: Move this to different, JVM-specific module?
+object JvmDeclarationCheckers : DeclarationCheckers() {
+    override val basicDeclarationCheckers: Set<FirBasicDeclarationChecker> = setOf(
+        FirJvmExternalDeclarationChecker,
+    )
+}

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/session/FirSessionFactory.kt

@@ -22,6 +22,7 @@ import org.jetbrains.kotlin.fir.analysis.extensions.additionalCheckers
 import org.jetbrains.kotlin.fir.caches.FirCachesFactory
 import org.jetbrains.kotlin.fir.caches.FirThreadUnsafeCachesFactory
 import org.jetbrains.kotlin.fir.checkers.registerCommonCheckers
+import org.jetbrains.kotlin.fir.checkers.registerJvmCheckers
 import org.jetbrains.kotlin.fir.extensions.BunchOfRegisteredExtensions
 import org.jetbrains.kotlin.fir.extensions.extensionService
 import org.jetbrains.kotlin.fir.extensions.registerExtensions
@@ -94,6 +95,7 @@ object FirSessionFactory {
 
             FirSessionConfigurator(this).apply {
                 registerCommonCheckers()
+                registerJvmCheckers()
                 init()
             }.configure()