[ADT] Allow llvm::hash_code as DenseMap key.

Summary:
This is useful when hash collisions are unlikely and acceptable, e.g. in clangd
completion ranking.

Reviewers: ioeric

Subscribers: ilya-biryukov, llvm-commits

Differential Revision: https://reviews.llvm.org/D48361

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335125 91177308-0d34-0410-b5e6-96231b3b80d8