[clangd] show layout info when hovering on a class/field definition.

Summary:
This triggers only on the definition itself, not on references (probably too
noisy). Inspecting the definition seems like a decent hint for being interested
in layout.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77355