Add equality to diagnostic context, simplifying deduplication

compiler/frontend.common/src/org/jetbrains/kotlin/KtSourceElement.kt

@@ -193,6 +193,21 @@ sealed class KtFakeSourceElementKind : KtSourceElementKind() {
 sealed class AbstractKtSourceElement {
     abstract val startOffset: Int
     abstract val endOffset: Int
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other !is AbstractKtSourceElement) return false
+
+        if (startOffset != other.startOffset) return false
+        if (endOffset != other.endOffset) return false
+
+        return true
+    }
+
+    override fun hashCode(): Int {
+        var result = startOffset
+        result = 31 * result + endOffset
+        return result
+    }
 }
 
 class KtOffsetsOnlySourceElement(

compiler/frontend.common/src/org/jetbrains/kotlin/diagnostics/DiagnosticReporter.kt

@@ -67,5 +67,23 @@ open class KtDiagnosticReporterWithContext(
         ) {
             sourceElement?.let { report(factory.on(it, a, positioningStrategy), this) }
         }
+
+        override fun equals(other: Any?): Boolean {
+            if (this === other) return true
+            if (other !is DiagnosticContextImpl) return false
+
+            if (sourceElement != other.sourceElement) return false
+            if (containingFilePath != other.containingFilePath) return false
+            if (languageVersionSettings != other.languageVersionSettings) return false
+
+            return true
+        }
+
+        override fun hashCode(): Int {
+            var result = sourceElement?.hashCode() ?: 0
+            result = 31 * result + containingFilePath.hashCode()
+            result = 31 * result + languageVersionSettings.hashCode()
+            return result
+        }
     }
 }

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/JvmSignatureClashDetector.kt

@@ -147,9 +147,10 @@ class JvmSignatureClashDetector(
         irDeclarations: Collection<IrDeclaration>,
         conflictingJvmDeclarationsData: ConflictingJvmDeclarationsData
     ) {
-        for (irDeclaration in irDeclarations) {
+        irDeclarations.mapNotNullTo(LinkedHashSet()) { irDeclaration ->
             context.ktDiagnosticReporter.atFirstValidFrom(irDeclaration, irClass, containingIrFile = irDeclaration.file)
-                .report(diagnosticFactory1, conflictingJvmDeclarationsData)
+        }.forEach {
+            it.report(diagnosticFactory1, conflictingJvmDeclarationsData)
         }
     }