[FIR] Extract FirPluginKey into :core:compiler.common and rename it to GeneratedDeclarationKey

This is needed to avoid dependency on :compiler:fir:fir2ir module in
  backend parts of compiler plugins. It will allow to publish those
  parts into jars for IDE, where they are needed for working of debugger
  and bytecode toolwindow

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/IrDeclarationDeserializer.kt

@@ -771,7 +771,7 @@ class IrDeclarationDeserializer(
     companion object {
         private val allKnownDeclarationOrigins = IrDeclarationOrigin::class.nestedClasses.toList()
         private val declarationOriginIndex =
-            allKnownDeclarationOrigins.map { it.objectInstance as IrDeclarationOriginImpl }.associateBy { it.name }
+            allKnownDeclarationOrigins.mapNotNull { it.objectInstance as? IrDeclarationOriginImpl }.associateBy { it.name }
     }
 
     private fun deserializeIrDeclarationOrigin(protoName: Int): IrDeclarationOriginImpl {
@@ -821,4 +821,4 @@ class IrDeclarationDeserializer(
             else -> false
         }
     }
-}
+}