Merge pull request #76399 from al45tair/eng/PR-135787913

[Backtracer][macOS] Fix crashing thread index when Rosetta is in use.

Author: al45tair

stdlib/public/libexec/swift-backtrace/TargetMacOS.swift

@@ -249,11 +249,12 @@ class Target {
 
       if info.thread_id == crashingThread {
         ctx = HostContext.fromHostMContext(mcontext)
-        crashingThreadNdx = Int(ndx)
+        crashingThreadNdx = threads.count
       } else {
         guard let threadCtx = HostContext.fromHostThread(ports[Int(ndx)]) else {
           // This can happen legitimately, e.g. when looking at a Rosetta 2
           // process, where there are ARM64 threads AS WELL AS the x86_64 ones.
+          mach_port_deallocate(mach_task_self_, ports[Int(ndx)])
           continue
         }
         ctx = threadCtx