[clangd] Expose value of enumerators to Hover API. (not UI yet)

Summary: This is part of https://github.com/clangd/clangd/issues/180.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70209