[IR] Fixed bug with thread unsafety

There is no need in a singleton here

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/ir/IrUtils.kt

@@ -466,7 +466,7 @@ val IrFunction.allParametersCount: Int
 // This is essentially the same as FakeOverrideBuilder,
 // but it bypasses SymbolTable.
 // TODO: merge it with FakeOverrideBuilder.
-private object FakeOverrideBuilderForLowerings : FakeOverrideBuilderStrategy() {
+private class FakeOverrideBuilderForLowerings : FakeOverrideBuilderStrategy() {
 
     override fun linkFunctionFakeOverride(declaration: IrFakeOverrideFunction) {
         declaration.acquireSymbol(IrSimpleFunctionSymbolImpl(WrappedSimpleFunctionDescriptor()))
@@ -491,7 +491,7 @@ private object FakeOverrideBuilderForLowerings : FakeOverrideBuilderStrategy() {
 }
 
 fun IrClass.addFakeOverrides(irBuiltIns: IrBuiltIns, implementedMembers: List<IrOverridableMember> = emptyList()) {
-    IrOverridingUtil(irBuiltIns, FakeOverrideBuilderForLowerings)
+    IrOverridingUtil(irBuiltIns, FakeOverrideBuilderForLowerings())
         .buildFakeOverridesForClassUsingOverriddenSymbols(this, implementedMembers)
         .forEach { addChild(it) }
 }