Multi-platform fix: no more errors for top-level impl fun / val / var in IDE

idea/testData/multiModuleHighlighting/platform5/header/header.kt

@@ -0,0 +1,3 @@
+header fun foo(): Int
+
+header val bar: String

idea/testData/multiModuleHighlighting/platform5/jvm/jvm.kt

@@ -0,0 +1,3 @@
+impl fun foo() = 42
+
+impl val bar = "Hello"

idea/tests/org/jetbrains/kotlin/idea/caches/resolve/MultiModuleHighlightingTest.kt

@@ -112,4 +112,16 @@ class MultiModuleHighlightingTest : AbstractMultiModuleHighlightingTest() {
 
         checkHighlightingInAllFiles()
     }
+
+    fun testPlatform5() {
+        val header = module("header")
+        header.setPlatformKind(TargetPlatformKind.Default)
+
+        val jvm = module("jvm")
+        jvm.setPlatformKind(TargetPlatformKind.Jvm.JVM_1_6)
+        jvm.enableMultiPlatform()
+        jvm.addDependency(header)
+
+        checkHighlightingInAllFiles()
+    }
 }