[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

llvm-svn: 308662