[clang-tools-extra] clangd: Add a build option to disable building dexp (PR #133124)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

@kraj, you approved this and you are a project member, are you comfortable merging this? If so, please go ahead.

>From my point of view as a clangd maintainer, I'm supportive of having such an option, but I have ~zero CMake knowledge, so if you think this needs more review, we need to find and add another reviewer.

https://github.com/llvm/llvm-project/pull/133124