[clangd] Fix remote index build without shared libs mode

Summary:
Generated Protobuf library has to be in CLANG_EXPORTS and should also be
installed appropriately. The easiest way to do that is via CMake's
add_clang_library. That unfortunately applies "one directory - one
clang_(library|tool)" policy so .proto files should be in a separate directory
and complicates the layout.

This setup works both in shared and static libs mode.

Resolves: https://github.com/clangd/clangd/issues/351

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: mgorny, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D78885