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

compiler/config.jvm/src/org/jetbrains/kotlin/config/JVMConfigurationKeys.java

@@ -156,4 +156,7 @@ public class JVMConfigurationKeys {
 
     public static final CompilerConfigurationKey<Boolean> LINK_VIA_SIGNATURES =
             CompilerConfigurationKey.create("Link JVM IR symbols via signatures, instead of by descriptors");
+
+    public static final CompilerConfigurationKey<Boolean> ENABLE_DEBUG_MODE =
+            CompilerConfigurationKey.create("Enable debug mode");
 }