[clangd] Don't use zlib when it's unavailable.

Without this patch `clangd` crashes at try to load compressed string table when `zlib` is not available.
Example:
- Build `clangd` with MinGW (`zlib` found)
- Build index
- Build `clangd` with Visual Studio compiler (`zlib` not found)
- Try to load index

Reviewed By: sammccall, adamcz

Differential Revision: https://reviews.llvm.org/D87673