[Runtime] Improve the metadata hashing function.

The inputs to the hash function is pointers that have a predictable patten. The
hashes that we were generating and using for the metadata caches were not very
good, and as a result we generated very deep search trees. A small change that
improved the utilization of the 'length' field and another bit-rotate round
improved the quality of the hash function.

I am going to attach to the github commit two pictures. The first picture is the
binary with the old hash. The first tree is very deep and sparse. The second
picture is with the new hash function and the tree is very wide and uniform.  I
used the benchmark 'TypeFlood' in debug mode to generate huge amounts of
metadata.

stdlib/public/runtime/MetadataCache.h

@@ -66,12 +66,13 @@ class EntryRef {
  }

  size_t hash() {
    size_t H = 0x56ba80d1 ^ length ;
    size_t H = 0x56ba80d1 * length ;
    for (unsigned i = 0; i < length; i++) {
      H = (H >> 10) | (H << ((sizeof(size_t) * 8) - 10));
      H ^= ((size_t)args[i]) ^ ((size_t)args[i] >> 19);
    }
    return H * 0x27d4eb2d;
    H *= 0x27d4eb2d;
    return (H >> 10) | (H << ((sizeof(size_t) * 8) - 10));
  }

  const void * const *begin() const { return args; }