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/safeCalls/assignSafeCall.dot

@@ -34,19 +34,9 @@ digraph assignSafeCall_kt {
     8 -> {9} [style=dotted];
 
     subgraph cluster_3 {
-        color=red
-        10 [label="Enter property" style="filled" fillcolor=red];
-        11 [label="Const: Int(1)"];
-        12 [label="Exit property" style="filled" fillcolor=red];
-    }
-    10 -> {11};
-    11 -> {12};
-    12 -> {19} [color=green];
-
-    subgraph cluster_4 {
         color=red
         13 [label="Enter function bar" style="filled" fillcolor=red];
-        subgraph cluster_5 {
+        subgraph cluster_4 {
             color=blue
             14 [label="Enter block"];
             15 [label="Exit block"];
@@ -57,16 +47,25 @@ digraph assignSafeCall_kt {
     14 -> {15};
     15 -> {16};
 
-    subgraph cluster_6 {
+    subgraph cluster_5 {
         color=red
         17 [label="Enter class A" style="filled" fillcolor=red];
         18 [label="Part of class initialization"];
+        subgraph cluster_6 {
+            color=blue
+            10 [label="Enter property" style="filled" fillcolor=red];
+            11 [label="Const: Int(1)"];
+            12 [label="Exit property" style="filled" fillcolor=red];
+        }
         19 [label="Exit class A" style="filled" fillcolor=red];
     }
     17 -> {18} [color=green];
     18 -> {10} [color=green];
     18 -> {19} [style=dotted];
     18 -> {10} [style=dashed];
+    10 -> {11};
+    11 -> {12};
+    12 -> {19} [color=green];
 
     subgraph cluster_7 {
         color=red