Move CHECK stack traces to be before the message (#1216)

Before, if the llvm hook to print a stack trace on CHECK was bound, it would trace after the message rather than before the message. I'm thinking swapping the order will generally be easier to debug.

common/check_test.cpp

@@ -14,6 +14,8 @@ TEST(CheckTest, CheckTrue) { CHECK(true); }
 TEST(CheckTest, CheckFalse) {
   // TODO: figure out why we can't use \\d+ instead of .+ in these patterns.
   ASSERT_DEATH({ CHECK(false); },
+               "Stack trace:\n"
+               ".+\n"
                "CHECK failure at common/check_test.cpp:.+: false\n");
 }