[Macros] Mitigate plugin process 'wait' failure

Previously, the compiler waited the plugin process with
`llvm::sys::Wait(pid, /*SecondsToWait=*/1)`. But in the `Wait`
implementation, it sets the `alarm(SecondsToWait)`, then
`wait4(pid, ..)` so if the alarm fires before the `wait4` call,
it may miss the timeout and can wait indefinitely. To mitigate that
risk,use `10` for the timeout value.

rdar://150474701

Author: rintaro

lib/AST/PluginRegistry.cpp

@@ -219,8 +219,10 @@ LoadedExecutablePlugin::PluginProcess::~PluginProcess() {
   // that. Usually when the pipe is closed above, the plugin detects the EOF in
   // the stdin and exits immediately, so this usually doesn't wait for the
   // timeout. Note that we can't use '0' because it performs a non-blocking
-  // wait, which make the plugin a zombie if it hasn't exited.
-  llvm::sys::Wait(process, /*SecondsToWait=*/1);
+  // wait, which make the plugin a zombie if it hasn't exited. We don't use
+  // a small number like '1' because the timeout alarm(3) can fire before the
+  // wait4(2).
+  llvm::sys::Wait(process, /*SecondsToWait=*/10);
 }
 
 ssize_t LoadedExecutablePlugin::PluginProcess::read(void *buf,