[XRay][compiler-rt] Use gettid instead of getpid in FDR mode.

[dberris]

Summary

This was pointed out that FDR mode didn't quite put the thread ID in the
buffers, but instead would write down the parent process ID.

compiler-rt/trunk/lib/xray/xray_fdr_logging.cc

 //
 //===----------------------------------------------------------------------===//
 #include "xray_fdr_logging.h"
 #include <algorithm>
 #include <bitset>
 #include <cassert>
 #include <cstring>
 #include <memory>
+#include <sys/syscall.h>
 #include <sys/time.h>
 #include <time.h>
 #include <unistd.h>
 #include <unordered_map>
 
 #include "sanitizer_common/sanitizer_common.h"
 #include "xray/xray_interface.h"
 #include "xray/xray_records.h"
std::aligned_storage<sizeof(MetadataRecord)>::type Records[InitRecordsCount];
     // Write out a MetadataRecord to signify that this is the start of a new
     // buffer, associated with a particular thread, with a new CPU.  For the
     // data, we have 15 bytes to squeeze as much information as we can.  At this
     // point we only write down the following bytes:
     //   - Thread ID (pid_t, 4 bytes)
     auto &NewBuffer = *reinterpret_cast<MetadataRecord *>(&Records[0]);
     NewBuffer.Type = RecordType::Metadata;
     NewBuffer.RecordKind = MetadataRecord::RecordKinds::NewBuffer;
-    pid_t Pid = getpid();
-    std::memcpy(&NewBuffer.Data, &Pid, sizeof(pid_t));
+    pid_t Tid = syscall(SYS_gettid);
+    std::memcpy(&NewBuffer.Data, &Tid, sizeof(pid_t));
   }
 
   // Also write the WalltimeMarker record.
   {
     static_assert(sizeof(time_t) <= 8, "time_t needs to be at most 8 bytes");
     auto &WalltimeMarker = *reinterpret_cast<MetadataRecord *>(&Records[1]);
     WalltimeMarker.Type = RecordType::Metadata;
     WalltimeMarker.RecordKind = MetadataRecord::RecordKinds::WalltimeMarker;