[ADT] Move CachedHashString to its own header in ADT, and rename to CachedHashStringRef.

Summary:
Reclaiming the name 'CachedHashString' will let us add a type with that
name that owns its value.

Reviewers: timshen

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25644

llvm-svn: 284434