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

[akyrtzi]

Summary

add_llvm_library() would cause the proto target to be added in LLVMExports.cmake and could fail an external configuration
of compiler-rt with error "The following imported targets are referenced, but are missing: grpc++ protobuf::libprotobuf"

[steven_wu]

LGTM

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}")
 
-  add_llvm_library(${LibraryName} ${GeneratedProtoSource}
+  set(LIBTYPE STATIC)
+  if(NOT XCODE)
+    # The Xcode generator doesn't handle object libraries correctly.
+    list(APPEND LIBTYPE OBJECT)
+  endif()
+  llvm_add_library(${LibraryName} ${LIBTYPE} ${GeneratedProtoSource}
     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