Has actual (new MPP): handle possible actual in "common" module

Before this commit, we assumed all actual declarations are
in platform modules, and each platform has no more than one actual.
In new MPP, actual declarations can be also in common code,
and each platform as well as common code can include
more than one actual declarations for given expected.

#KT-26217 Fixed

idea/src/org/jetbrains/kotlin/idea/highlighter/markers/HasActualMarker.kt

@@ -19,29 +19,34 @@ package org.jetbrains.kotlin.idea.highlighter.markers
 import com.intellij.codeInsight.daemon.impl.PsiElementListNavigator
 import com.intellij.ide.util.DefaultPsiElementCellRenderer
 import com.intellij.psi.PsiElement
-import org.jetbrains.kotlin.descriptors.MemberDescriptor
-import org.jetbrains.kotlin.idea.caches.project.implementingDescriptors
-import org.jetbrains.kotlin.idea.caches.resolve.findModuleDescriptor
 import org.jetbrains.kotlin.idea.core.toDescriptor
 import org.jetbrains.kotlin.idea.util.actualsForExpected
-import org.jetbrains.kotlin.idea.util.hasActualsFor
 import org.jetbrains.kotlin.psi.KtDeclaration
 import org.jetbrains.kotlin.resolve.MultiTargetPlatform
+import org.jetbrains.kotlin.resolve.descriptorUtil.module
 import org.jetbrains.kotlin.resolve.getMultiTargetPlatform
 import java.awt.event.MouseEvent
 
 fun getPlatformActualTooltip(declaration: KtDeclaration?): String? {
-    val descriptor = declaration?.toDescriptor() as? MemberDescriptor ?: return null
-    val commonModuleDescriptor = declaration.containingKtFile.findModuleDescriptor()
+    val actualDeclarations = declaration?.actualsForExpected() ?: return null
+    if (actualDeclarations.isEmpty()) return null
 
-    val platformModulesWithActuals = commonModuleDescriptor.implementingDescriptors.filter {
-        it.hasActualsFor(descriptor)
-    }
-    if (platformModulesWithActuals.isEmpty()) return null
-
-    return platformModulesWithActuals.joinToString(prefix = "Has actuals in ") {
-        (it.getMultiTargetPlatform() as MultiTargetPlatform.Specific).platform
-    }
+    return actualDeclarations.asSequence()
+        .mapNotNull { it.toDescriptor()?.module }
+        .groupBy { it.getMultiTargetPlatform() }
+        .filter { (platform, _) -> platform != null }
+        .entries
+        .joinToString(prefix = "Has actuals in ") { (platform, modules) ->
+            val modulesSuffix = if (modules.size <= 1) "" else " (${modules.size} modules)"
+            when (platform) {
+                is MultiTargetPlatform.Specific ->
+                    "${platform.platform}$modulesSuffix"
+                MultiTargetPlatform.Common ->
+                    "common$modulesSuffix"
+                null ->
+                    throw AssertionError("Platform should not be null")
+            }
+        }
 }
 
 fun navigateToPlatformActual(e: MouseEvent?, declaration: KtDeclaration?) {