FIR: Fix bug with incorrect source element for qualifiers

idea/idea-fir/tests/org/jetbrains/kotlin/idea/resolve/FirReferenceResolveTestGenerated.java

@@ -144,6 +144,11 @@ public class FirReferenceResolveTestGenerated extends AbstractFirReferenceResolv
         runTest("idea/testData/resolve/references/EnumValues.kt");
     }
 
+    @TestMetadata("ExternalCompanionObject.kt")
+    public void testExternalCompanionObject() throws Exception {
+        runTest("idea/testData/resolve/references/ExternalCompanionObject.kt");
+    }
+
     @TestMetadata("FakeJavaLang1.kt")
     public void testFakeJavaLang1() throws Exception {
         runTest("idea/testData/resolve/references/FakeJavaLang1.kt");

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/references/FirReferenceResolveHelper.kt

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.idea.references
 
 import com.intellij.psi.tree.TokenSet
 import org.jetbrains.kotlin.fir.FirSession
+import org.jetbrains.kotlin.fir.ROOT_PREFIX_FOR_IDE_RESOLUTION_MODE
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.synthetic.FirSyntheticProperty
 import org.jetbrains.kotlin.fir.expressions.*
@@ -121,6 +122,7 @@ internal object FirReferenceResolveHelper {
             else -> {
                 qualified
                     .collectDescendantsOfType<KtSimpleNameExpression>()
+                    .dropWhile { it.getReferencedName() == ROOT_PREFIX_FOR_IDE_RESOLUTION_MODE }
                     .joinToString(separator = ".") { it.getReferencedName() }
                     .let(::FqName)
             }

idea/testData/resolve/references/ExternalCompanionObject.Data.kt

@@ -0,0 +1,5 @@
+package dependency
+
+class C {
+    companion object
+}

idea/testData/resolve/references/ExternalCompanionObject.kt

@@ -0,0 +1,5 @@
+fun usage() {
+    dependency.C.<caret>Companion
+}
+
+// REF: companion object of (dependency).C

idea/tests/org/jetbrains/kotlin/idea/resolve/ReferenceResolveTestGenerated.java

@@ -144,6 +144,11 @@ public class ReferenceResolveTestGenerated extends AbstractReferenceResolveTest
         runTest("idea/testData/resolve/references/EnumValues.kt");
     }
 
+    @TestMetadata("ExternalCompanionObject.kt")
+    public void testExternalCompanionObject() throws Exception {
+        runTest("idea/testData/resolve/references/ExternalCompanionObject.kt");
+    }
+
     @TestMetadata("FakeJavaLang1.kt")
     public void testFakeJavaLang1() throws Exception {
         runTest("idea/testData/resolve/references/FakeJavaLang1.kt");