[lldb] Make StackID comparison symmetric

Currently, if we're comparing a StackID on the stack with one StackID on the
heap, we perform a special kind of IsYounger comparison. However, if
`IsYounger(A, b)` returns true, it must be the case that `IsYounger(B, A)`
returns false. But this is not necessarily the case today because, before this
patch, we would fallback to the traditional "stack" comparison, which is wrong
if one of the StackIDs is on the heap.

Author: felipepiovezan

lldb/source/Target/StackID.cpp

@@ -86,13 +86,14 @@ IsYoungerHeapCFAs(const StackID &lhs, const StackID &rhs, Process &process) {
     return HeapCFAComparisonResult::NoOpinion;
 
   // FIXME: rdar://76119439
-  // At the boundary between an async parent frame calling a regular child
-  // frame, the CFA of the parent async function is a heap addresses, and the
-  // CFA of concrete child function is a stack address. Therefore, if lhs is
-  // on stack, and rhs is not, lhs is considered less than rhs, independent of
-  // address values.
+  // If one of the frames has a CFA on the stack and the other doesn't, we are
+  // at the boundary between an asynchronous and a synchronous function.
+  // Synchronous functions cannot call asynchronous functions, therefore the
+  // synchronous frame is always younger.
   if (lhs_cfa_on_stack && !rhs_cfa_on_stack)
     return HeapCFAComparisonResult::Younger;
+  if (!lhs_cfa_on_stack && rhs_cfa_on_stack)
+    return HeapCFAComparisonResult::Older;
   return HeapCFAComparisonResult::NoOpinion;
 }
 // END SWIFT

lldb/unittests/StackID/StackIDTest.cpp

@@ -86,8 +86,5 @@ TEST_F(StackIDTest, StackHeapCFAComparison) {
   MockStackID cfa_on_heap(/*cfa*/ 10, OnStack::No);
 
   EXPECT_TRUE(StackID::IsYounger(cfa_on_stack, cfa_on_heap, process));
-
-  // FIXME: if the above returned true, swapping the arguments **must** return
-  // false. And yet it doesn't.
-  // EXPECT_FALSE(StackID::IsYounger(cfa_on_heap, cfa_on_stack, process));
+  EXPECT_FALSE(StackID::IsYounger(cfa_on_heap, cfa_on_stack, process));
 }