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

test/cctest/test-mark-compact.cc

Index: test/cctest/test-mark-compact.cc
diff --git a/test/cctest/test-mark-compact.cc b/test/cctest/test-mark-compact.cc
index 5b7fe56133d128267a0d37972dcd129a12d92d38..ec70a4ca7b8641db4e2919d63988a5572aa85e44 100644
--- a/test/cctest/test-mark-compact.cc
+++ b/test/cctest/test-mark-compact.cc
@@ -423,7 +423,7 @@ static intptr_t MemoryInUse() {
 
   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.