[clangd] Fix remote index build on macOS

[kbobyrev]

Summary

macOS builds suddenly started failing:

https://github.com/kirillbobyrev/indexing-tools/runs/925090879

This patch makes use of imported libraries and fixes builds for macOS.

[kbobyrev]

Hmm, apparently link_libraries is not supposed to be used this way as it adds to linker search path _after_ it's called (from the docs) but this still worked for me on mac before. Hence I was relying on incorrect behaviour, my bad.

llvm/cmake/modules/FindGRPC.cmake

if (NOT HOMEBREW-NOTFOUND)
       execute_process(COMMAND ${HOMEBREW} --prefix protobuf
         OUTPUT_VARIABLE PROTOBUF_HOMEBREW_PATH
         RESULT_VARIABLE PROTOBUF_HOMEBREW_RETURN_CODE
         OUTPUT_STRIP_TRAILING_WHITESPACE)
       # If either library is not installed via Homebrew, fall back to the
       # system path.
       if (GRPC_HOMEBREW_RETURN_CODE EQUAL "0")
         include_directories(${GRPC_HOMEBREW_PATH}/include)
-        link_directories(${GRPC_HOMEBREW_PATH}/lib)
+        find_library(GRPC_LIBRARY
+                     grpc++
+                     PATHS ${GRPC_HOMEBREW_PATH}/lib
+                     NO_DEFAULT_PATH
+                     REQUIRED)
+        add_library(grpc++ UNKNOWN IMPORTED GLOBAL)
+        set_target_properties(grpc++ PROPERTIES
+                              IMPORTED_LOCATION ${GRPC_LIBRARY})
       endif()
       if (PROTOBUF_HOMEBREW_RETURN_CODE EQUAL "0")
         include_directories(${PROTOBUF_HOMEBREW_PATH}/include)
-        link_directories(${PROTOBUF_HOMEBREW_PATH}/lib)
+        find_library(PROTOBUF_LIBRARY
+                     protobuf
+                     PATHS ${PROTOBUF_HOMEBREW_PATH}/lib
+                     NO_DEFAULT_PATH
+                     REQUIRED)
+        add_library(protobuf UNKNOWN IMPORTED GLOBAL)
+        set_target_properties(protobuf PROPERTIES
+                              IMPORTED_LOCATION ${PROTOBUF_LIBRARY})
       endif()
     endif()
   endif()
 endif()
 
 # Proto headers are generated in ${CMAKE_CURRENT_BINARY_DIR}.
 # Libraries that use these headers should adjust the include path.
 # FIXME(kirillbobyrev): Allow optional generation of gRPC code and give callers