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 https://github.com/rust-lang/rust/issues/46437

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1955,7 +1955,6 @@ unsafe extern "C" {
         NameLen: size_t,
         T: &'a Type,
     ) -> &'a Value;
-    pub(crate) fn LLVMRustInsertPrivateGlobal<'a>(M: &'a Module, T: &'a Type) -> &'a Value;
     pub(crate) fn LLVMRustGetNamedValue(
         M: &Module,
         Name: *const c_char,