Ruby: Avoid computing full fastTC for AstNode::getParent

DIL before
```
                                                   /* AST::AstNode */ AST#87953007::Cached::TAstNode result) =
  fastTC(AST#a6718388::AstNode::getAChild#0#dispred#ff/2)
.

Synthesis#d9ff06b1::Desugared::getADescendant#0#dispred#ff(/* Synthesis::Desugared */ AST#87953007::Cached::TAstNode this,
                                                           /* AST::AstNode */ AST#87953007::Cached::TAstNode result)
:-
  (
    exists(int arg1,
           /* AST::AstNode */ dontcare AST#87953007::Cached::TAstNode _ |
      arg1 = -1, AST#87953007::Cached::getSynthChild#2(_, arg1, this)
    ),
    result = this
  );
  (
    exists(int arg1,
           /* AST::AstNode */ dontcare AST#87953007::Cached::TAstNode _ |
      arg1 = -1, AST#87953007::Cached::getSynthChild#2(_, arg1, this)
    ),
    #AST#a6718388::AstNode::getAChild#0#dispredPlus#ff(this, result)
  )
.
```

DIL after
```
incremental
Synthesis#d9ff06b1::Desugared::getADescendant#ff(/* Synthesis::Desugared */ AST#87953007::Cached::TAstNode this,
                                                 /* AST::AstNode */ AST#87953007::Cached::TAstNode result)
:-
  (
    exists(int arg1,
           /* AST::AstNode */ dontcare AST#87953007::Cached::TAstNode _ |
      arg1 = -1, AST#87953007::Cached::getSynthChild#2(_, arg1, this)
    ),
    result = this
  );
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode call_result#2 |
    exists(int arg1,
           /* AST::AstNode */ dontcare AST#87953007::Cached::TAstNode _ |
      arg1 = -1, AST#87953007::Cached::getSynthChild#2(_, arg1, this)
    ),
    rec Synthesis#d9ff06b1::Desugared::getADescendant#ff(this, call_result#2),
    exists(cached dontcare string _ |
      AST#a6718388::AstNode::getAChild#1#dispred(call_result#2, _, result)
    )
  )
| [base_case]
  exists(int arg1, /* AST::AstNode */ dontcare AST#87953007::Cached::TAstNode _ |
    arg1 = -1, AST#87953007::Cached::getSynthChild#2(_, arg1, this)
  ),
  result = this
| [delta_order]
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode call_result#2 |
    exists(int arg1,
           /* AST::AstNode */ dontcare AST#87953007::Cached::TAstNode _ |
      arg1 = -1, AST#87953007::Cached::getSynthChild#2(_, arg1, this)
    ),
    delta previous rec Synthesis#d9ff06b1::Desugared::getADescendant#ff(this,
                                                                        call_result#2),
    project#AST#a6718388::AstNode::getAChild#1#dispred(call_result#2, result)
  ),
  not(
    previous rec Synthesis#d9ff06b1::Desugared::getADescendant#ff(this, result)
  )
| [delta_order_up_to_500000]
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode call_result#2 |
    delta previous rec Synthesis#d9ff06b1::Desugared::getADescendant#ff(this,
                                                                        call_result#2),
    exists(int arg1,
           /* AST::AstNode */ dontcare AST#87953007::Cached::TAstNode _ |
      arg1 = -1, AST#87953007::Cached::getSynthChild#2(_, arg1, this)
    ),
    project#AST#a6718388::AstNode::getAChild#1#dispred(call_result#2, result)
  ),
  not(
    previous rec Synthesis#d9ff06b1::Desugared::getADescendant#ff(this, result)
  )
.
```

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

@@ -118,7 +118,11 @@ class Synthesis extends TSynthesis {
 private class Desugared extends AstNode {
   Desugared() { this = any(AstNode sugar).getDesugared() }
 
-  AstNode getADescendant() { result = this.getAChild*() }
+  AstNode getADescendant() {
+    result = this
+    or
+    result = this.getADescendant().getAChild()
+  }
 }
 
 /**