Revert "[FindGRPC.cmake] Use `llvm_add_library()` for the custom proto target instead of `add_llvm_library()`"

[akyrtzi]

Summary

Breaks configuring clangd with -DCLANGD_ENABLE_REMOTE=On (https://github.com/llvm/llvm-project/issues/57213)

This reverts commit ba688024077c3e51c6a0949246b2a1b76284e681.

[sammccall]

LG, thanks for unbreaking build.
Sorry I don't really have a clue about the imports/exports :-(

llvm/cmake/modules/FindGRPC.cmake

if (PROTO_GRPC)
     list(APPEND GeneratedProtoHeader "${CMAKE_CURRENT_BINARY_DIR}/${Basename}.grpc.pb.h")
   endif()
   add_custom_command(
         OUTPUT ${GeneratedProtoSource} ${GeneratedProtoHeader}
         COMMAND ${PROTOC}
         ARGS ${Flags} "${ProtoSourceAbsolutePath}"
         DEPENDS "${ProtoSourceAbsolutePath}")
 
-  set(LIBTYPE STATIC)
-  if(NOT XCODE)
-    # The Xcode generator doesn't handle object libraries correctly.
-    list(APPEND LIBTYPE OBJECT)
-  endif()
-  llvm_add_library(${LibraryName} ${LIBTYPE} ${GeneratedProtoSource}
+  add_llvm_library(${LibraryName} ${GeneratedProtoSource}
     PARTIAL_SOURCES_INTENDED
     LINK_LIBS PUBLIC grpc++ protobuf)
 
   # Ensure dependency headers are generated before dependent protos are built.
   # DEPENDS arg is a list of "Foo.proto". While they're logically relative to
   # the source dir, the generated headers we need are in the binary dir.
   foreach(ImportedProto IN LISTS PROTO_DEPENDS)
     # Foo.proto -> Foo.pb.h