Hashing: use 64-bit seed for hashing on all platforms.

get_execution_seed returns a size_t which varies across platforms, but its
users actually always feed it into a uint64_t role so it makes sense to be
consistent.

Mostly this is just a tidy-up, but it also apparently allows PCH files to be
shared between Clang compilers built for 32-bit and 64-bit hosts.

llvm-svn: 341113