[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


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@308067 91177308-0d34-0410-b5e6-96231b3b80d8