[clangd] Extend hints for gRPC and Protobuf CMake config paths

[kbobyrev]

Summary

Otherwise CMake can't find respective libraries in Ubuntu 20.04 causing our
builds to fail:

https://github.com/clangd/clangd/runs/1678096007

Resolves: https://github.com/clangd/clangd/issues/641

[sammccall]

(What broke this?)

[kbobyrev]

As mentioned offline, this is not the right fix :( The correct one is pushed to our CI.

llvm/cmake/modules/FindGRPC.cmake

 # FIXME(kirillbobyrev): Check if gRPC and Protobuf headers can be included at
 # configure time.
 find_package(Threads REQUIRED)
 if (GRPC_INSTALL_PATH)
   # This setup requires gRPC to be built from sources using CMake and installed
   # to ${GRPC_INSTALL_PATH} via -DCMAKE_INSTALL_PREFIX=${GRPC_INSTALL_PATH}.
   # Libraries will be linked according to gRPC build policy which generates
   # static libraries when BUILD_SHARED_LIBS is Off and dynamic libraries when
   # it's On (NOTE: This is a variable passed to gRPC CMake build invocation,
   # LLVM's BUILD_SHARED_LIBS has no effect).
   set(protobuf_MODULE_COMPATIBLE TRUE)
-  find_package(Protobuf CONFIG REQUIRED HINTS ${GRPC_INSTALL_PATH})
+  find_package(Protobuf CONFIG REQUIRED HINTS ${GRPC_INSTALL_PATH}/lib/cmake/protobuf)
   message(STATUS "Using protobuf ${protobuf_VERSION}")
-  find_package(gRPC CONFIG REQUIRED HINTS ${GRPC_INSTALL_PATH})
+  find_package(gRPC CONFIG REQUIRED HINTS ${GRPC_INSTALL_PATH}/lib/cmake/grpc)
   message(STATUS "Using gRPC ${gRPC_VERSION}")
 
   include_directories(${Protobuf_INCLUDE_DIRS})
 
   # gRPC CMake CONFIG gives the libraries slightly odd names, make them match
   # the conventional system-installed names.
   set_target_properties(protobuf::libprotobuf PROPERTIES IMPORTED_GLOBAL TRUE)
   add_library(protobuf ALIAS protobuf::libprotobuf)