Add a key to enable spilling of all variables in a suspending context

This commit adds a new key that will allow users to enhance their
debugging experience in suspending contexts when using the IR backend.
After the key is enabled, the following things are changed:
1. All variables in a suspending context are spilled regardless their
liveness.
2. Their LVT records are not shrunk.
3. ACONST_NULL is not spilled to dead variables.

#KT-48678 In progress

(cherry picked from commit 38d97d0621)

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JVMCompilerArguments.kt

@@ -511,6 +511,13 @@ Also sets `-jvm-target` value equal to the selected JDK version"""
     )
     var linkViaSignatures: Boolean by FreezableVar(false)
 
+    @Argument(
+        value = "-Xdebug",
+        description = "Enable debug mode for compilation.\n" +
+                "Currently this includes spilling all variables in a suspending context regardless their liveness."
+    )
+    var enableDebugMode: Boolean by FreezableVar(false)
+
     override fun configureAnalysisFlags(collector: MessageCollector, languageVersion: LanguageVersion): MutableMap<AnalysisFlag<*>, Any> {
         val result = super.configureAnalysisFlags(collector, languageVersion)
         result[JvmAnalysisFlags.strictMetadataVersionSemantics] = strictMetadataVersionSemantics