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/analysis-tests/testData/resolve/cfg/annotatedLocalClass.dot

@@ -57,6 +57,12 @@ digraph annotatedLocalClass_kt {
         23 [label="Enter class Local" style="filled" fillcolor=red];
         24 [label="Exit class Local" style="filled" fillcolor=red];
     }
+    subgraph cluster_8 {
+        color=blue
+        25 [label="Enter function <init>" style="filled" fillcolor=red];
+        26 [label="Delegated constructor call: super<R|kotlin/Any|>()" style="filled" fillcolor=yellow];
+        27 [label="Exit function <init>" style="filled" fillcolor=red];
+    }
     5 -> {6};
     6 -> {7};
     7 -> {8};
@@ -79,13 +85,6 @@ digraph annotatedLocalClass_kt {
     20 -> {21};
     21 -> {22};
     23 -> {24} [color=green];
-
-    subgraph cluster_8 {
-        color=red
-        25 [label="Enter function <init>" style="filled" fillcolor=red];
-        26 [label="Delegated constructor call: super<R|kotlin/Any|>()" style="filled" fillcolor=yellow];
-        27 [label="Exit function <init>" style="filled" fillcolor=red];
-    }
     25 -> {26};
     26 -> {27};