[XRay][compiler-rt] Explicitly initialise members.

Before this, the change committed in D26232 might have an uninitialised
std::atomic<bool> that may or may not have a valid state. On aarch64
this breaks consistently, while it doesn't manifest as a problem in
x86_64.

This is an attempt to un-break this in aarch64.

llvm-svn: 288776

compiler-rt/lib/xray/xray_buffer_queue.cc

@@ -19,7 +19,7 @@
 using namespace __xray;
 
 BufferQueue::BufferQueue(std::size_t B, std::size_t N)
-    : BufferSize(B), Buffers(N) {
+    : BufferSize(B), Buffers(N), Mutex(), OwnedBuffers(), Finalizing(false) {
   for (auto &Buf : Buffers) {
     void *Tmp = malloc(BufferSize);
     Buf.Buffer = Tmp;