Scripting: fix descriptor extraction, restore main-kts testing

Temporary solution for K1 - see comments in the code
it seems that a scenario reproduced in the MainKtsTest.testKt48812
wasn't covered with analogous changes before.
And it wasn't detected while main-kts tests weren't a part of the
K1 scripting tests.
The test is fixed now, and testing of main-kts is restored.

build.gradle.kts

@@ -708,6 +708,7 @@ tasks {
         dependsOn(":kotlin-scripting-jsr223-test:embeddableTest")
         dependsOn(":kotlin-scripting-ide-services-test:test")
         dependsOn(":kotlin-scripting-ide-services-test:embeddableTest")
+        dependsOn(":kotlin-main-kts-test:test")
     }
 
     register("scriptingK2Test") {

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/descriptors/IrBasedDescriptors.kt

@@ -132,6 +132,7 @@ fun IrDeclaration.toIrBasedDescriptor(): DeclarationDescriptor = when (this) {
     is IrField -> toIrBasedDescriptor()
     is IrTypeAlias -> toIrBasedDescriptor()
     is IrErrorDeclaration -> toIrBasedDescriptor()
+    is IrScript -> toIrBasedDescriptor()
     else -> error("Unknown declaration kind")
 }
 
@@ -1260,7 +1261,11 @@ private fun IrSimpleFunctionSymbol.toIrBasedDescriptorIfPossible(): FunctionDesc
 private fun IrPropertySymbol.toIrBasedDescriptorIfPossible(): PropertyDescriptor =
     if (isBound) owner.toIrBasedDescriptor() else descriptor
 
-// this is a temporary solution for scripts - seems that introducing full-blown emulation of descriptors for the single degenerate case
-// doesn't make any sense.
+// this is a temporary solution for scripts
+// TODO: implement IR-based descriptors for scripts, see KT-60631
 @OptIn(ObsoleteDescriptorBasedAPI::class)
 private fun IrScriptSymbol.toIrBasedDescriptorIfPossible(): ScriptDescriptor = descriptor
+
+// see comment above to IrScriptSymbol.toIrBasedDescriptorIfPossible()
+@OptIn(ObsoleteDescriptorBasedAPI::class)
+private fun IrScript.toIrBasedDescriptor() = descriptor