[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.
- 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