Suggest %zu for size_t args to printf.

Suggest %zu for size_t args to printf.

For PR11152. Make PrintSpecifier::fixType() suggest "%zu" for size_t, etc.
rather than looking at the underlying type and suggesting "%llu" or other
platform-specific length modifiers. Applies to C99 and C++11.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@142342 91177308-0d34-0410-b5e6-96231b3b80d8