[clang] Fix handling of "%zd" in scanf

[clang] Fix handling of "%zd" in scanf

This diff addresses FIXMEs in lib/Analysis/ScanfFormatString.cpp
for the case of ssize_t format specifier and adds tests.
In particular, this change enables Clang to emit a warning
on incorrect using of "%zd"/"%zn".

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D35652

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@308662 91177308-0d34-0410-b5e6-96231b3b80d8