Fix a couple of incorrect format string warnings

This patch fixes use of incorrect `%zi` to format a plain `int`, and using
`%llu` to format a `uint64_t`. The fix is to use the new typesafe
`llvm::Formatv` based API.

Differential Revision: https://reviews.llvm.org/D28028
Subscribers: lldb-commits

llvm-svn: 290359