[IR] Made signature nullable (#3860)

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

@@ -36,7 +36,7 @@ abstract class GlobalDeclarationTable(
     protected fun loadKnownBuiltins(builtIns: IrBuiltIns) {
         builtIns.knownBuiltins.forEach {
             val symbol = (it as IrSymbolOwner).symbol
-            table[it] = symbol.signature.also { id -> clashTracker.commit(it, id) }
+            table[it] = symbol.signature!!.also { id -> clashTracker.commit(it, id) }
         }
     }
 

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

@@ -119,7 +119,7 @@ class CurrentModuleWithICDeserializer(
         symbolTable.forEachPublicSymbol {
             if (it.descriptor.isDirtyDescriptor()) { // public && non-deserialized should be dirty symbol
                 if (it !in knownBuiltIns) {
-                    dirtyDeclarations[it.signature] = it
+                    dirtyDeclarations[it.signature!!] = it
                 }
             }
         }

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

@@ -37,9 +37,10 @@ abstract class IrModuleDeserializer(val moduleDescriptor: ModuleDescriptor) {
     abstract fun deserializeIrSymbol(idSig: IdSignature, symbolKind: BinarySymbolData.SymbolKind): IrSymbol
 
     open fun declareIrSymbol(symbol: IrSymbol) {
-        assert(symbol.isPublicApi) { "Symbol is not public API: ${symbol.descriptor}" }
+        val signature = symbol.signature
+        require(signature != null) { "Symbol is not public API: ${symbol.descriptor}" }
         assert(symbol.descriptor !is WrappedDeclarationDescriptor<*>)
-        deserializeIrSymbol(symbol.signature, symbol.kind())
+        deserializeIrSymbol(signature, symbol.kind())
     }
 
     open val klib: IrLibrary get() = error("Unsupported operation")
@@ -166,8 +167,9 @@ class IrModuleDeserializerWithBuiltIns(
     }
 
     override fun declareIrSymbol(symbol: IrSymbol) {
-        if (symbol.isPublicApi && checkIsFunctionInterface(symbol.signature))
-            resolveFunctionalInterface(symbol.signature, symbol.kind())
+        val signature = symbol.signature
+        if (signature != null && checkIsFunctionInterface(signature))
+            resolveFunctionalInterface(signature, symbol.kind())
         else delegate.declareIrSymbol(symbol)
     }