[clangd][Hover] Suppress initializers with many tokens

This results in excessive memory usage and eats a lot of screen estate.
Especially in the cases with lots of nested macro calls.

This patch tries to remedy it before the release cut by suppressing the
initializers. For better UX we should probably update the expression printer to
truncate those (behind some policy).

Fixes https://github.com/clangd/clangd/issues/917

Differential Revision: https://reviews.llvm.org/D118260