[CMake] Update FindGPRC to use add_llvm_library

[CMake] Update FindGPRC to use add_llvm_library

add_clang_library is not available in components other than clang and
clang-tool-extras. Trying to use this module elsewhere will cause cmake
error. `add_clang_library` doesn't seem necessary for clangd's GRPC.
Change it to a more generic `add_llvm_library` so it is easier to build
other downstream projects with GRPC that doesn't depend on clang.

Reviewed By: akyrtzi

Differential Revision: https://reviews.llvm.org/D131593