[clang-tools-extra] [clangd] Implement fold range for #pragma region (PR #168177)

[Haojian Wu via cfe-commits]

hokein wrote:

> By the way @hokein, unrelated to this patch but would you be able to help with next steps discussed [here](https://github.com/clangd/clangd/pull/2445#issuecomment-3247594120)?

Yeah, I made a reply there.

https://github.com/llvm/llvm-project/pull/168177