Use CachedHashStringRef instead of CachedHash<StringRef>.

Summary:
This uses one less word on 64-bit platforms, so should be a strict
improvement.  This change also lets us get rid of llvm::CachedHash.

Reviewers: rafael, timshen

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25646

llvm-svn: 284502