[lldb/Reproducer] Disable buffering of stdout during replay

Different buffering behavior during capture and replay caused some of
the shell tests to fail when run from a reproducer. By disabling stdout
buffering we get a better approximation of how things get flushed during
an regular debug session. There is a performance impact but since this
only affects replay this is acceptable.

lldb/source/Utility/ReproducerInstrumentation.cpp

@@ -8,6 +8,7 @@
 
 #include "lldb/Utility/ReproducerInstrumentation.h"
 #include "lldb/Utility/Reproducer.h"
+#include <stdio.h>
 
 using namespace lldb_private;
 using namespace lldb_private::repro;
@@ -47,6 +48,10 @@ bool Registry::Replay(llvm::StringRef buffer) {
   Log *log = GetLogIfAllCategoriesSet(LIBLLDB_LOG_API);
 #endif
 
+  // Disable buffering stdout so that we approximate the way things get flushed
+  // during an interactive session.
+  setvbuf(stdout, nullptr, _IONBF, 0);
+
   Deserializer deserializer(buffer);
   while (deserializer.HasData(1)) {
     unsigned id = deserializer.Deserialize<unsigned>();