FIR CFA: properly visit subgraphs in checkers

Interpretation: a graph A is a subgraph of B if information available at
nodes of A depends on the paths taken in B. For example, local classes
are subgraphs of a graph in which they are declared, and members of
those classes are subgraphs of the local class itself - because these
members can reference captured values.

Consequences:

 * if graph G is a subgraph of node N, then G is a subgraph of N's
   owner;
 * `ControlFlowAnalysisDiagnosticComponent` will only visit root graphs;
 * `graph.traverse` will ignore subgraph boundaries, as if all subgraphs
   are inlined into one huge root graph;
 * if a control flow checker needs information from a declaration to
   which a graph is attached, it must look at subgraphs explicitly.

For example, consider the `callsInPlace` checker. When a function
has a `callsInPlace` contract and a local declaration, the checker must
visit that local declaration to ensure it does not capture the allegedly
called-in-place argument - hence `graph.traverse` will look at the
nodes. However, the local declaration can also be a function with its
own `callsInPlace` contracts, so the checker should also run for it in
isolation. If that sounds quadratic, that's because unfortunately it is.

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/ControlFlowGraphBuilder.kt

@@ -433,6 +433,7 @@ class ControlFlowGraphBuilder {
             } ?: continue
 
             createPartOfClassInitializationNode(declaration as FirControlFlowGraphOwner).also {
+                currentGraph.addSubGraph(graph)
                 addEdge(node, it, preferredKind = EdgeKind.CfgForward)
                 addEdge(it, graph.enterNode, preferredKind = EdgeKind.CfgForward)
                 node = graph.exitNode
@@ -537,6 +538,7 @@ class ControlFlowGraphBuilder {
             if (functionGraph != null && functionGraph.owner == null) {
                 addEdge(node, functionGraph.enterNode, preferredKind = EdgeKind.CfgForward)
                 node.addSubGraph(functionGraph)
+                currentGraph.addSubGraph(functionGraph)
             }
         }
     }