JVM_IR: make continuation detection more consistent

* unify various checks for whether a suspend function needs a
    continuation;

  * mark the continuation parameter with an origin (this also allows
    correctly computing the offset of the local in codegen -- see the
    new test);

  * when wrapping `suspend fun main`, use a suspend reference instead of
    a synthetic non-suspend lambda (required to correctly implement the
    previous point, as previously the continuation parameter was passed
    to main() itself correctly only because of very lenient checks in
    AddContinuationLowering).

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

@@ -185,7 +185,7 @@ fun IrTypeParameter.copyToWithoutSuperTypes(
     }
 }
 
-private fun IrFunction.copyReceiverParametersFrom(from: IrFunction) {
+fun IrFunction.copyReceiverParametersFrom(from: IrFunction) {
     dispatchReceiverParameter = from.dispatchReceiverParameter?.let {
         IrValueParameterImpl(it.startOffset, it.endOffset, it.origin, it.descriptor, it.type, it.varargElementType).also {
             it.parent = this
@@ -200,22 +200,6 @@ fun IrFunction.copyValueParametersFrom(from: IrFunction) {
     valueParameters += from.valueParameters.map { it.copyTo(this, index = it.index + shift) }
 }
 
-fun IrFunction.copyValueParametersInsertingContinuationFrom(from: IrFunction, insertContinuation: () -> Unit) {
-    copyReceiverParametersFrom(from)
-    val shift = valueParameters.size
-    var additionalShift = 0
-    from.valueParameters.forEach {
-        // The continuation parameter goes before the default argument mask and handler.
-        if (it.origin == IrDeclarationOrigin.MASK_FOR_DEFAULT_FUNCTION && additionalShift == 0) {
-            insertContinuation()
-            additionalShift = 1
-        }
-        valueParameters += it.copyTo(this, index = it.index + shift + additionalShift)
-    }
-    // If there was no default argument mask and handler, the continuation goes last.
-    if (additionalShift == 0) insertContinuation()
-}
-
 fun IrFunction.copyParameterDeclarationsFrom(from: IrFunction) {
     assert(typeParameters.isEmpty())
     copyTypeParametersFrom(from)