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.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1954,7 +1954,6 @@ 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,