Use the ugly PRIx64 macro to make format string portable.

This is debugging code so functionality isn't a concern, but mingw32 warns
because it doesn't understand the %llx format specifier.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180024 91177308-0d34-0410-b5e6-96231b3b80d8