[codegeneration][debug info] enchance debugger behaviour for paramterless when

- generates tramboline like a11b07f6c3 on -Xg-generate-when-marker=enable
- fixes issue KT-33364

kotlin-native/backend.native/cli.bc/src/org/jetbrains/kotlin/cli/bc/K2Native.kt

@@ -178,6 +178,15 @@ class K2Native : CLICompiler<K2NativeCompilerArguments>() {
                         null
                     }
                 })
+                putIfNotNull(GENERATE_WHEN_MARKER, when (val it = arguments.generateWhenMarkerString) {
+                    "enable" -> true
+                    "disable" -> false
+                    null -> null
+                    else -> {
+                        configuration.report(ERROR, "Unsupported -Xg-generate-when-marker= value: $it. Possible values are 'enable'/'disable'")
+                        null
+                    }
+                })
                 put(STATIC_FRAMEWORK, selectFrameworkType(configuration, arguments, outputKind))
                 put(OVERRIDE_CLANG_OPTIONS, arguments.clangOptions.toNonNullList())
                 put(ALLOCATION_MODE, arguments.allocator)

kotlin-native/backend.native/cli.bc/src/org/jetbrains/kotlin/cli/bc/K2NativeCompilerArguments.kt

@@ -171,6 +171,12 @@ class K2NativeCompilerArguments : CommonCompilerArguments() {
             description = """generates marker of inlined function body on call site to make debugger breakpoint resolution more accurate"""
     )
     var generateInlinedFunctionMarkerString: String? = null
+    @Argument(
+            value = "-Xg-generate-when-marker",
+            valueDescription = "{disable|enable}",
+            description = """generates marker of when to make debugger breakpoint resolution more accurate"""
+    )
+    var generateWhenMarkerString: String? = null
 
 
     @Argument(

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/KonanConfig.kt

@@ -44,6 +44,7 @@ class KonanConfig(val project: Project, val configuration: CompilerConfiguration
     val lightDebug: Boolean = configuration.get(KonanConfigKeys.LIGHT_DEBUG)
             ?: target.family.isAppleFamily // Default is true for Apple targets.
     val generateInlinedBodyTrampoline = debug && configuration.get(KonanConfigKeys.GENERATE_INLINED_FUNCTION_BODY_MARKER) ?: false
+    val generateWhenTrampoline = debug && configuration.get(KonanConfigKeys.GENERATE_WHEN_MARKER) ?: false
 
     val memoryModel: MemoryModel get() = configuration.get(KonanConfigKeys.MEMORY_MODEL)!!
     val destroyRuntimeMode: DestroyRuntimeMode get() = configuration.get(KonanConfigKeys.DESTROY_RUNTIME_MODE)!!

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/KonanConfigurationKeys.kt

@@ -58,6 +58,8 @@ class KonanConfigKeys {
                 = CompilerConfigurationKey.create("add light debug information")
         val GENERATE_INLINED_FUNCTION_BODY_MARKER: CompilerConfigurationKey<Boolean?>
                 = CompilerConfigurationKey.create("generates inlined function body marker on call site")
+        val GENERATE_WHEN_MARKER: CompilerConfigurationKey<Boolean?>
+                = CompilerConfigurationKey.create("generates when marker on call site")
         val LINKER_ARGS: CompilerConfigurationKey<List<String>>
                 = CompilerConfigurationKey.create("additional linker arguments")
         val LIST_PHASES: CompilerConfigurationKey<Boolean>

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/llvm/IrToBitcode.kt

@@ -1177,6 +1177,12 @@ internal class CodeGeneratorVisitor(val context: Context, val lifetimes: Map<IrE
 
         val whenEmittingContext = WhenEmittingContext(expression)
 
+        val generationContext = (currentCodeContext.functionScope() as? FunctionScope)?.functionGenerationContext
+                .takeIf { context.config.generateWhenTrampoline }
+        generationContext?.basicBlock("when", expression.startLocation)?.let {
+            generationContext.br(it)
+            generationContext.positionAtEnd(it)
+        }
         expression.branches.forEach {
             val bbNext = if (it == expression.branches.last())
                              null