Automerge: [lldb] AIX Changes for MainLoop polling (#120378)

This PR is in reference to porting LLDB on AIX.

Link to discussions on llvm discourse and github:

1. https://discourse.llvm.org/t/port-lldb-to-ibm-aix/80640
2. llvm/llvm-project#101657
The complete changes for porting are present in this draft PR:
llvm/llvm-project#102601

Dropping changes for MainLoop polling in AIX, as `ppoll` is not
supported in AIX currently.
This change is part of the couple of minimal changes required to build a
minimal `lldb` binary on AIX

lldb/source/Host/posix/MainLoopPosix.cpp

@@ -99,6 +99,7 @@ class MainLoopPosix::RunImpl {
   ~RunImpl() = default;
 
   Status Poll();
+
   void ProcessReadEvents();
 
 private:
@@ -159,6 +160,22 @@ MainLoopPosix::RunImpl::RunImpl(MainLoopPosix &loop) : loop(loop) {
   read_fds.reserve(loop.m_read_fds.size());
 }
 
+static int StartPoll(llvm::MutableArrayRef<struct pollfd> fds,
+                     std::optional<MainLoopPosix::TimePoint> point) {
+#if HAVE_PPOLL
+  return ppoll(fds.data(), fds.size(), ToTimeSpec(point),
+               /*sigmask=*/nullptr);
+#else
+  using namespace std::chrono;
+  int timeout = -1;
+  if (point) {
+    nanoseconds dur = std::max(*point - steady_clock::now(), nanoseconds(0));
+    timeout = ceil<milliseconds>(dur).count();
+  }
+  return poll(fds.data(), fds.size(), timeout);
+#endif
+}
+
 Status MainLoopPosix::RunImpl::Poll() {
   read_fds.clear();
 
@@ -169,11 +186,9 @@ Status MainLoopPosix::RunImpl::Poll() {
     pfd.revents = 0;
     read_fds.push_back(pfd);
   }
+  int ready = StartPoll(read_fds, loop.GetNextWakeupTime());
 
-  if (ppoll(read_fds.data(), read_fds.size(),
-            ToTimeSpec(loop.GetNextWakeupTime()),
-            /*sigmask=*/nullptr) == -1 &&
-      errno != EINTR)
+  if (ready == -1 && errno != EINTR)
     return Status(errno, eErrorTypePOSIX);
 
   return Status();