Fix for PR4192: fix the definition of int64_t on x86_64 Linux.

Fix for PR4192: fix the definition of int64_t on x86_64 Linux.

Note that I'm guessing that *BSD and Solaris do the same thing as Linux
here, but it's quite possible I'm wrong; if the following testcase
gives an error on x86-64 with gcc for any of those operating systems, please
tell me:

#include <stdint.h>

int64_t x; long x;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@74583 91177308-0d34-0410-b5e6-96231b3b80d8