[clangd] Enhance macro hover to see full definition

Summary: Signed-off-by: Marc-Andre Laperle <malaperle@gmail.com>

Reviewers: simark, ilya-biryukov, sammccall, ioeric, hokein

Reviewed By: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D55250

llvm-svn: 354761