[IR] Extract descriptor part from SymbolTable into separate component

After this change SymbolTable (and ReferenceSymbolTable) contains only
  methods with IdSignatures. All descriptors-related methods are moved
  into DescriptorSymbolTableExtension, which automatically delegates to
  the SymbolTable if needed

At this moment there are cross-references between SymbolTable, because
  descriptor API is still actively used across backends. So SymbolTable
  is accessible in some place then descriptor extension will be accessible
  too

DescriptorSymbolTableExtension is an implementation of abstract SymbolTableExtension
  which allows to implement different kinds of storages, e.g. FIR based
  (it probably will be needed for FIR2IR)

compiler/tests-common-new/tests/org/jetbrains/kotlin/test/backend/handlers/IrInlineBodiesHandler.kt

@@ -7,12 +7,12 @@ package org.jetbrains.kotlin.test.backend.handlers
 
 import org.jetbrains.kotlin.ir.IrElement
 import org.jetbrains.kotlin.ir.declarations.IrExternalPackageFragment
+import org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI
 import org.jetbrains.kotlin.ir.declarations.IrSimpleFunction
 import org.jetbrains.kotlin.ir.declarations.lazy.AbstractIrLazyFunction
 import org.jetbrains.kotlin.ir.expressions.IrMemberAccessExpression
 import org.jetbrains.kotlin.ir.util.DeserializableClass
 import org.jetbrains.kotlin.ir.util.IdSignature
-import org.jetbrains.kotlin.ir.util.allUnbound
 import org.jetbrains.kotlin.ir.util.resolveFakeOverride
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitorVoid
 import org.jetbrains.kotlin.ir.visitors.acceptChildrenVoid
@@ -24,13 +24,14 @@ import org.jetbrains.kotlin.test.services.TestServices
 class IrInlineBodiesHandler(testServices: TestServices) : AbstractIrHandler(testServices) {
     val declaredInlineFunctionSignatures = mutableSetOf<IdSignature>()
 
+    @OptIn(ObsoleteDescriptorBasedAPI::class)
     override fun processModule(module: TestModule, info: IrBackendInput) {
         info.processAllIrModuleFragments(module) { irModule, _ ->
             irModule.acceptChildrenVoid(InlineFunctionsCollector())
             irModule.acceptChildrenVoid(InlineCallBodiesCheck(firEnabled = module.frontendKind == FrontendKinds.FIR))
         }
 
-        assertions.assertTrue((info as IrBackendInput.JvmIrBackendInput).backendInput.symbolTable.allUnbound.isEmpty())
+        assertions.assertTrue((info as IrBackendInput.JvmIrBackendInput).backendInput.symbolTable.descriptorExtension.allUnboundSymbols.isEmpty())
     }
 
     override fun processAfterAllModules(someAssertionWasFailed: Boolean) {