Rollup merge of #147488 - AMS21:remove_llvm_rust_insert_private_global, r=nikic

refactor: Remove `LLVMRustInsertPrivateGlobal` and `define_private_global`

Since it can easily be implemented using the existing LLVM C API in
terms of `LLVMAddGlobal` and `LLVMSetLinkage` and `define_private_global`
was only used in one place.

Work towards rust-lang/rust#46437

Author: Zalathar