K2: report DUPLICATE_CLASS_NAMES error in JVM backend

It was already reported in the K2+PSI mode, but not LT because
BuilderFactoryForDuplicateClassNameDiagnostics relied on PSI, and did
not do anything if PSI was missing.

No tests were added because it fixes the already existing test
`compiler/testData/cli/jvm/fileClassClashMultipleFiles` after the
project is migrated to 2.0.

 #KT-59586

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/diagnostics/JvmBackendErrors.kt

@@ -39,6 +39,8 @@ object JvmBackendErrors {
 
     val NOT_ALL_MULTIFILE_CLASS_PARTS_ARE_JVM_SYNTHETIC by error0<PsiElement>()
 
+    val DUPLICATE_CLASS_NAMES by error2<PsiElement, String, String>()
+
     init {
         RootDiagnosticRendererFactory.registerFactory(KtDefaultJvmErrorMessages)
     }
@@ -78,5 +80,7 @@ object KtDefaultJvmErrorMessages : BaseDiagnosticRendererFactory() {
             JvmBackendErrors.NOT_ALL_MULTIFILE_CLASS_PARTS_ARE_JVM_SYNTHETIC,
             "All of multi-file class parts should be annotated with @JvmSynthetic if at least one of them is"
         )
+
+        map.put(JvmBackendErrors.DUPLICATE_CLASS_NAMES, "Duplicate JVM class name ''{0}'' generated from: {1}", STRING, STRING)
     }
 }

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/diagnostics/JvmDeclarationOrigin.kt

@@ -25,6 +25,11 @@ open class JvmDeclarationOrigin(
     val descriptor: DeclarationDescriptor?,
     val parametersForJvmOverload: List<KtParameter?>? = null
 ) {
+    // This property is used to get the original element in the sources, from which this declaration was generated.
+    // In the old JVM backend, it is just the PSI element. In JVM IR, it is the original IR element (before any deep copy).
+    open val originalSourceElement: Any?
+        get() = element
+
     override fun toString(): String =
         if (this == NO_ORIGIN) "NO_ORIGIN" else "origin=$originKind element=${element?.javaClass?.simpleName} descriptor=$descriptor"