[XRay] [compiler-rt] stop writing garbage in naive log records

[pelikan]

Summary

Turns out sizeof(packed) isn't as strong as we'd hoped. This makes sure
that when we initialize the padding, all 12 bytes will be zero.

[dberris]

LGTM

[pelikan]

sync

compiler-rt/trunk/include/xray/xray_records.h

struct alignas(32) XRayRecord {
 
   // Get the full 8 bytes of the TSC when we get the log record.
   uint64_t TSC = 0;
 
   // The thread ID for the currently running thread.
   uint32_t TId = 0;
 
   // Use some bytes in the end of the record for buffers.
-  char Buffer[4] = {};
+  char Buffer[12] = {};
 } __attribute__((packed));
 
 static_assert(sizeof(XRayRecord) == 32, "XRayRecord != 32 bytes");
 
 struct alignas(32) XRayArgPayload {
   // We use the same 16 bits as a discriminant for the records in the log here
   // too, and so that the first 4 bytes are packed properly.
   uint16_t RecordType = RecordTypes::ARG_PAYLOAD;