[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 an embarrassing bug in how we computed the offset of
a iterator in the vector:

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

Author: felipepiovezan

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

@@ -2584,9 +2584,9 @@ 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 (8) rows of the plan and store FP's location.
+  const int upper_bound = std::min(8, unwind_plan.GetRowCount());
+  llvm::SmallVector<AbstractRegisterLocation, 8> fp_locs;
   for (int row_idx = 0; row_idx < upper_bound; row_idx++) {
     RowSP row = unwind_plan.GetRowAtIndex(row_idx);
     AbstractRegisterLocation regloc;
@@ -2613,7 +2613,7 @@ 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.