Change getSlotIndex to return unsigned.

The actual storage was already using unsigned, but the interface was using
uint64_t. This is wasteful on 32 bits and looks to be the root causes of
a miscompilation on Windows where a value was being sign extended to 64bits
to compare with the result of getSlotIndex.

Patch by Pasi Parviainen!

llvm-svn: 180791