[clangd] Show hower info for expressions

Summary:
This currently populates only the Name with the expression's type and
Value if expression is evaluatable.

Fixes https://github.com/clangd/clangd/issues/56

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72500