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/smartcasts/stability/overridenOpenVal.dot

@@ -13,25 +13,24 @@ digraph overridenOpenVal_kt {
     1 -> {2};
 
     subgraph cluster_1 {
-        color=red
-        3 [label="Enter property" style="filled" fillcolor=red];
-        4 [label="Access variable R|<local>/x|"];
-        5 [label="Exit property" style="filled" fillcolor=red];
-    }
-    3 -> {4};
-    4 -> {5};
-    5 -> {8} [color=green];
-
-    subgraph cluster_2 {
         color=red
         6 [label="Enter class A" style="filled" fillcolor=red];
         7 [label="Part of class initialization"];
+        subgraph cluster_2 {
+            color=blue
+            3 [label="Enter property" style="filled" fillcolor=red];
+            4 [label="Access variable R|<local>/x|"];
+            5 [label="Exit property" style="filled" fillcolor=red];
+        }
         8 [label="Exit class A" style="filled" fillcolor=red];
     }
     6 -> {7} [color=green];
     7 -> {3} [color=green];
     7 -> {8} [style=dotted];
     7 -> {3} [style=dashed];
+    3 -> {4};
+    4 -> {5};
+    5 -> {8} [color=green];
 
     subgraph cluster_3 {
         color=red