Switch to ssize_t from size_t to unbreak windows builders.

Builders that have -fms-compatibility on by default define size_t implicitly.
Tests that provide conflicting definitions would cause unintended failures.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199195 91177308-0d34-0410-b5e6-96231b3b80d8