[clangd] Add Value field to HoverInfo

Summary:
Put a symbols value information which is deduced from initializer
expression into HoverInfo struct.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63330

llvm-svn: 364390