compiler: Forward attributes to eii-expanded macros

Otherwise you get errors like these if you have an Externally
Implementable Item defined in std:

    error: attribute macro has missing stability attribute
        --> library/std/src/io/mod.rs:2269:1
         |
    2269 | #[eii(on_broken_pipe)]
         | ^^^^^^^^^^^^^^^^^^^^--
         | |
         | in this attribute macro expansion
         |
        ::: library/core/src/macros/mod.rs:1899:5
         |
    1899 |     pub macro eii($item:item) {
         |     ------------- in this expansion of `#[eii]`

Or (fatal) warnings like these:

    warning: missing documentation for an attribute macro
        --> library/std/src/io/mod.rs:2269:1

Author: Enselic

compiler/rustc_builtin_macros/src/eii.rs

@@ -133,6 +133,7 @@ fn eii_(
         macro_name,
         foreign_item_name,
         impl_unsafe,
+        &attrs_from_decl,
     )));
 
     return_items.into_iter().map(wrap_item).collect()
@@ -416,9 +417,14 @@ fn generate_attribute_macro_to_implement(
     macro_name: Ident,
     foreign_item_name: Ident,
     impl_unsafe: bool,
+    attrs_from_decl: &[Attribute],
 ) -> ast::Item {
     let mut macro_attrs = ThinVec::new();
 
+    // To avoid e.g. `error: attribute macro has missing stability attribute`
+    // errors for eii's in std.
+    macro_attrs.extend_from_slice(attrs_from_decl);
+
     // #[builtin_macro(eii_shared_macro)]
     macro_attrs.push(ecx.attr_nested_word(sym::rustc_builtin_macro, sym::eii_shared_macro, span));