[clang] Use -Wshorten-64-to-32 to enable warnings about 64bit to 32bit truncations.

test/cctest/test-mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 405 matching lines @@
 // increase even though the sum of the allocated object sizes decreases.  It
 // also means that the memory use depends on the kernel and stdlib.
 static intptr_t MemoryInUse() {
   intptr_t memory_use = 0;
 
   int fd = open("/proc/self/maps", O_RDONLY);
   if (fd < 0) return -1;
 
   const int kBufSize = 10000;
   char buffer[kBufSize];
-  int length = read(fd, buffer, kBufSize);
+  ssize_t length = read(fd, buffer, kBufSize);
   intptr_t line_start = 0;
   CHECK_LT(length, kBufSize);  // Make the buffer bigger.
   CHECK_GT(length, 0);  // We have to find some data in the file.
   while (line_start < length) {
     if (buffer[line_start] == '\n') {
       line_start++;
       continue;
     }
     intptr_t position = line_start;
     uintptr_t start = ReadLong(buffer, &position, 16);
@@ skipping 46 matching lines @@
 
 
 TEST(RegressJoinThreadsOnIsolateDeinit) {
   intptr_t size_limit = ShortLivingIsolate() * 2;
   for (int i = 0; i < 10; i++) {
     CHECK_GT(size_limit, ShortLivingIsolate());
   }
 }
 
 #endif  // __linux__ and !USE_SIMULATOR