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

[Khem Raj via cfe-commits]

kraj 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.

@HighCommander4 thanks, this has been tested in yocto autobuilder for few days already and I am happy with the change.

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