[clangd] Add Hover.MacroContentsLimit config option (#155105)

Currently macro expansions are hard capped at 2048. This PR adds
the `Hover.MacroContentsLimit` config option for overriding the default.

Fixes https://github.com/llvm/llvm-project/issues/153355

GitOrigin-RevId: a0cc776732980e792b201e3848c1049dfc3b1836
7 files changed