FIR DFA: split Flow into PersistentFlow and MutableFlow

The distinction is similar to persistent data structures and their
builders. Only the MutableFlow can be passed to LogicSystem for
modification; when it's ready, it can be converted into PersistentFlow
and attached to a CFG node.

The result is that the API is cleaner, the implementation is a bit more
neat, and hopefully the use of PersistentHashMap.Builder improves
performance a little. Also the node-to-flow map can now be removed in
favor of just storing PersistentFlow inside a node, seeing as it's
explicitly immutable and all that.

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/resolver/resolverUtils.kt

@@ -47,9 +47,9 @@ internal open class StubBodyResolveTransformerComponents(
     transformer,
     context,
 ) {
-    override val dataFlowAnalyzer: FirDataFlowAnalyzer<*>
-        get() = object : FirDataFlowAnalyzer<PersistentFlow>(this@StubBodyResolveTransformerComponents, context.dataFlowAnalyzerContext) {
-            override val logicSystem: LogicSystem<PersistentFlow>
+    override val dataFlowAnalyzer: FirDataFlowAnalyzer
+        get() = object : FirDataFlowAnalyzer(this@StubBodyResolveTransformerComponents, context.dataFlowAnalyzerContext) {
+            override val logicSystem: LogicSystem
                 get() = error("Should not be called")
 
             override val receiverStack: Iterable<ImplicitReceiverValue<*>>