[lldb][swift] Increase number of Rows inspected when unwinding async functions

We were overly conservatives in our initial estimations: if callee saved
registers are used, they are usually spilled first, increasing the
number of rows in the prologue.

This also revealed a bug in computing the offset of a vector iterator:

```
-  int row_idx = fp_locs.end() - it;
+  int row_idx = it - fp_locs.begin();
```

Testing this is unfortunately difficult: a way of forcing register
spilling is required. For now, this commit tweaks one of the existing
tests in a way that seems to reliably spill more registers, but there
are no guarantees this will continue to be the case. Other testing
mechanisms need to be created.

Author: felipepiovezan

lldb/source/Plugins/LanguageRuntime/Swift/SwiftLanguageRuntime.cpp

@@ -2584,9 +2584,13 @@ GetFrameSetupInfo(UnwindPlan &unwind_plan, RegisterContext &regctx) {
   if (!fp_unwind_regdomain)
     return fp_unwind_regdomain.takeError();
 
-  // Look at the first few (4) rows of the plan and store FP's location.
-  const int upper_bound = std::min(4, unwind_plan.GetRowCount());
-  llvm::SmallVector<AbstractRegisterLocation, 4> fp_locs;
+  // Look at the first few (12) rows of the plan and store FP's location.
+  // This number is based on AAPCS, with 10 callee-saved GPRs and 8 floating
+  // point registers. When STP instructions are used, the plan would have one
+  // initial row, nine rows of saving callee-saved registers, and two standard
+  // prologue rows (fp+lr and sp).
+  const int upper_bound = std::min(12, unwind_plan.GetRowCount());
+  llvm::SmallVector<AbstractRegisterLocation, 12> fp_locs;
   for (int row_idx = 0; row_idx < upper_bound; row_idx++) {
     RowSP row = unwind_plan.GetRowAtIndex(row_idx);
     AbstractRegisterLocation regloc;
@@ -2613,7 +2617,7 @@ GetFrameSetupInfo(UnwindPlan &unwind_plan, RegisterContext &regctx) {
   // Use subsequent row, if available.
   // Pointer auth may introduce more instructions, but they don't affect the
   // unwinder rows / store to the stack.
-  int row_idx = fp_locs.end() - it;
+  int row_idx = it - fp_locs.begin();
   int next_row_idx = row_idx + 1;
 
   // If subsequent row is invalid, approximate through current row.

lldb/test/API/lang/swift/async/stepping/step_over_lots_of_tasks/TestSwiftAsyncSteppingManyTasks.py

@@ -55,7 +55,7 @@ def test_step_over_top_level_fibonacci(self):
         fib_bp.SetEnabled(False)
 
         fib_first_line = thread_in_fib.frames[0].GetLineEntry().GetLine()
-        num_lines_fib = 5
+        num_lines_fib = 7
         for line_offset in range(1, num_lines_fib):
             thread_in_fib.StepOver()
             self.assertEqual(process.GetSelectedThread(), thread_in_fib)

lldb/test/API/lang/swift/async/stepping/step_over_lots_of_tasks/main.swift

@@ -12,13 +12,17 @@
 
 func fib(n: Int) async -> Int {
   if (n == 0) {
+    print("done!")
     return 1
   }
   if (n == 1) {
+    print("done!")
     return 1
   }
   async let n1_task = fib(n: n - 1)  // Breakpoint fib
   async let n2_task = fib(n: n - 2)
+  print(n)
+  if (n == 6) { do { try await Task.sleep(for: .seconds(1)) } catch {} }
   let n1 = await n1_task
   let n2 = await n2_task
   return n1 + n2