Ruby: Avoid computing full fastTC for AstNode::getParent

DIL before
```
                                  /* AST::AstNode */ AST#87953007::Cached::TAstNode result) =
  fastTC(Module#fe82a56b::parent#1#ff/2)
.

Module#fe82a56b::enclosingModule#1#ff(/* AST::AstNode */ AST#87953007::Cached::TAstNode node,
                                      /* Module::ModuleBase */ AST#87953007::Cached::TAstNode result)
:-
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode call_result#2 |
    Module#2a43f566::ModuleBase#f(result),
    project#AST#a6718388::AstNode::getAChild#1#dispred(result, call_result#2),
    (
      node = call_result#2;
      #Module#fe82a56b::parent#1Plus#ff(node, call_result#2)
    )
  )
.
```

DIL after
```
incremental
Module#fe82a56b::enclosingModule#1#ff(/* AST::AstNode */ AST#87953007::Cached::TAstNode node,
                                      /* Module::ModuleBase */ AST#87953007::Cached::TAstNode result)
:-
  (
    Module#2a43f566::ModuleBase#f(result),
    exists(cached dontcare string _ |
      AST#a6718388::AstNode::getAChild#1#dispred(result, _, node)
    )
  );
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode mid |
    Module#2a43f566::ModuleBase#f(result),
    rec Module#fe82a56b::enclosingModule#1#ff(mid, result),
    not(Module#2a43f566::ModuleBase#f(mid)),
    not(Method#8b49e67f::Block#f(mid)),
    exists(cached dontcare string _ |
      AST#a6718388::AstNode::getAChild#1#dispred(mid, _, node)
    )
  )
| [base_case]
  Module#2a43f566::ModuleBase#f(result),
  project#AST#a6718388::AstNode::getAChild#1#dispred(result, node)
| [delta_order]
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode mid |
    Module#2a43f566::ModuleBase#f(result),
    delta previous rec Module#fe82a56b::enclosingModule#1#ff(mid, result),
    not(Module#2a43f566::ModuleBase#f(mid)),
    not(Method#8b49e67f::Block#f(mid)),
    project#AST#a6718388::AstNode::getAChild#1#dispred(mid, node)
  ),
  not(previous rec Module#fe82a56b::enclosingModule#1#ff(node, result))
| [delta_order_up_to_500000]
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode mid |
    delta previous rec Module#fe82a56b::enclosingModule#1#ff(mid, result),
    Module#2a43f566::ModuleBase#f(result),
    not(Module#2a43f566::ModuleBase#f(mid)),
    not(Method#8b49e67f::Block#f(mid)),
    project#AST#a6718388::AstNode::getAChild#1#dispred(mid, node)
  ),
  not(previous rec Module#fe82a56b::enclosingModule#1#ff(node, result))
.
```

Author: hvitved

ruby/ql/lib/codeql/ruby/ast/internal/Module.qll

@@ -486,12 +486,15 @@ private import ResolveImpl
  * methods evaluate the block in the context of some other module/class instead of
  * the enclosing one.
  */
-private ModuleBase enclosingModule(AstNode node) { result = parent*(node).getParent() }
-
-private AstNode parent(AstNode n) {
-  result = n.getParent() and
-  not result instanceof ModuleBase and
-  not result instanceof Block
+private ModuleBase enclosingModule(AstNode node) {
+  result = node.getParent()
+  or
+  exists(AstNode mid |
+    result = enclosingModule(mid) and
+    mid = node.getParent() and
+    not mid instanceof ModuleBase and
+    not mid instanceof Block
+  )
 }
 
 private Module getAncestors(Module m) {