[clang] Fix handling of "%zd" format specifier

This diff addresses FIXME in lib/Analysis/PrintfFormatString.cpp
and makes PrintfSpecifier::getArgType return the correct type. 
In particular, this change enables Clang to emit a warning on 
incorrect using of "%zd"/"%zn" format specifiers.

Differential revision: https://reviews.llvm.org/D35427

Test plan: make check-all

llvm-svn: 308067