[llvm-exegesis] Replace size_t with ssize_t where relevant

Currently BenchmarkRunner.cpp stores the return code of recvmsg as
size_t. Not only is this incorrect (as recvmsg returns ssize_t), but it
also makes the error code check after the statement completely irrelvant
as it checks if the number of bytes read is greater than zero (which
will always be true for an unsigned type).

Author: boomanaiden154

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp

@@ -221,7 +221,7 @@ class SubProcessFunctionExecutorImpl
     Message.msg_control = ControlBuffer;
     Message.msg_controllen = sizeof(ControlBuffer);
 
-    size_t BytesRead = recvmsg(SocketFD, &Message, 0);
+    ssize_t BytesRead = recvmsg(SocketFD, &Message, 0);
 
     if (BytesRead < 0)
       return make_error<Failure>("Failed to read FD from socket");