[LLVM][ADT] Convert llvm::hash_code to unsigned explicitly in DenseMapInfo (#77743)

The getHashValue() signature returns a value of type 'unsigned' while
the hash_code could only be implicitly converted to 'size_t'. Depending
on the C++ implementation, this may or may not be a narrowing
conversion.

On some platform/compiler combination, this becomes a warning. To avoid
the warning (and better highlight the narrowing), do an explicit
conversion instead.

Co-authored-by: Orest Chura <orest.chura@intel.com>