Disable the ssize_t test in format-strings-fixit.c.

Turns out this part of the test from r142342 wasn't portable.
The errors on the bots look like this:

E:\bb-win7\cmake-clang-i686-msys\build\tools\clang\test\Sema\Output\format-strings-fixit.c.tmp:58:13: error: conversion specifies type 'unsigned int' but the argument has type 'ssize_t' (aka 'long')
  printf("%zd", (ssize_t) 42);
          ~~^   ~~~~~~~~~~~~
          %zd

Obviously we can't typedef ssize_t to someting that doesn't have the same size as size_t and expect it to work.

But it's also weird that the format string "%zd" gets interpreted as "unsigned int" when it should clearly be signed.

llvm-svn: 142345