Include debug info for the allocator shim

Issue Details:
In some cases it is necessary to generate an "allocator shim" to forward various Rust allocation functions (e.g., `__rust_alloc`) to an underlying function (e.g., `malloc`). However, since this allocator shim is a manually created LLVM module it is not processed via the normal module processing code and so no debug info is generated for it (if debugging info is enabled).

Fix Details:
* Modify the `debuginfo` code to allow creating debug info for a module without a `CodegenCx` (since it is difficult, and expensive, to create one just to emit some debug info).
* After creating the allocator shim add in basic debug info.

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -120,7 +120,8 @@ pub trait ExtraBackendMethods: CodegenBackend + WriteBackendMethods + Sized + Se
     fn codegen_allocator<'tcx>(
         &self,
         tcx: TyCtxt<'tcx>,
-        mods: &mut Self::Module,
+        module_llvm: &mut Self::Module,
+        module_name: &str,
         kind: AllocatorKind,
         has_alloc_error_handler: bool,
     );