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::isInDesugaredContext#1#f(/* AST::AstNode */ unique AST#87953007::Cached::TAstNode n)
:-
  exists(int arg1, /* AST::AstNode */ dontcare AST#87953007::Cached::TAstNode _ |
    arg1 = -1, AST#87953007::Cached::getSynthChild#2(_, arg1, n)
  );
  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, call_result#2)
    ),
    #AST#a6718388::AstNode::getAChild#0#dispredPlus#ff(call_result#2, n)
  )
.
```

DIL after
```
incremental
Synthesis#d9ff06b1::isInDesugaredContext#1#f(/* AST::AstNode */ unique AST#87953007::Cached::TAstNode n)
:-
  exists(int arg1, /* AST::AstNode */ dontcare AST#87953007::Cached::TAstNode _ |
    arg1 = -1, AST#87953007::Cached::getSynthChild#2(_, arg1, n)
  );
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode any#expr##2 |
    rec Synthesis#d9ff06b1::isInDesugaredContext#1#f(any#expr##2),
    exists(cached dontcare string _ |
      AST#a6718388::AstNode::getAChild#1#dispred(any#expr##2, _, n)
    )
  )
| [base_case]
  exists(int arg1, /* AST::AstNode */ dontcare AST#87953007::Cached::TAstNode _ |
    arg1 = -1, AST#87953007::Cached::getSynthChild#2(_, arg1, n)
  )
| [delta_order]
  exists(/* AST::AstNode */ AST#87953007::Cached::TAstNode any#expr##2 |
    delta previous rec Synthesis#d9ff06b1::isInDesugaredContext#1#f(any#expr##2),
    project#AST#a6718388::AstNode::getAChild#1#dispred(any#expr##2, n)
  ),
  not(previous rec Synthesis#d9ff06b1::isInDesugaredContext#1#f(n))
.
``

Author: hvitved

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

@@ -132,7 +132,10 @@ int desugarLevel(AstNode n) { result = count(Desugared desugared | n = desugared
  * Holds if `n` appears in a context that is desugared. That is, a
  * transitive, reflexive parent of `n` is a desugared node.
  */
-predicate isInDesugaredContext(AstNode n) { n = any(AstNode sugar).getDesugared().getAChild*() }
+predicate isInDesugaredContext(AstNode n) {
+  n = any(AstNode sugar).getDesugared() or
+  n = any(AstNode mid | isInDesugaredContext(mid)).getAChild()
+}
 
 /**
  * Holds if `n` is a node that only exists as a result of desugaring some