[FIR] Implement extension for providing additional type attributes

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/CompilerConeAttributes.kt

@@ -89,6 +89,11 @@ object CompilerConeAttributes {
         UnsafeVariance.ANNOTATION_CLASS_ID to UnsafeVariance
     )
 
+    val classIdByCompilerAttribute: Map<ConeAttribute<*>, ClassId> = compilerAttributeByClassId.entries.associateBy(
+        keySelector = { it.value },
+        valueTransform = { it.key }
+    )
+
     val compilerAttributeByFqName: Map<FqName, ConeAttribute<*>> = compilerAttributeByClassId.mapKeys { it.key.asSingleFqName() }
 }
 

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeAttributes.kt

@@ -25,7 +25,7 @@ abstract class ConeAttribute<T : ConeAttribute<T>> : AnnotationMarker {
      *
      * This function must be symmetrical: a.add(b) == b.add(a)
      */
-    abstract fun add(other: @UnsafeVariance T?): T
+    abstract fun add(other: @UnsafeVariance T?): T?
     abstract fun isSubtypeOf(other: @UnsafeVariance T?): Boolean
 
     abstract override fun toString(): String