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

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

`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"

Differential Revision: https://reviews.llvm.org/D132033