Add a CachedHash structure.

A DenseMap doesn't store the hashes, so it needs to recompute them when
the table is resized.

In some applications the hashing cost is noticeable. That is the case
for example in lld for symbol names (StringRef).

This patch adds a templated structure that can wraps any value that can
go in a DenseMap and caches the hash.

llvm-svn: 266981