Avoid more MetaItem-to-Attribute conversions.

There is code for converting `Attribute` (syntactic) to `MetaItem`
(semantic). There is also code for the reverse direction. The reverse
direction isn't really necessary; it's currently only used when
generating attributes, e.g. in `derive` code.

This commit adds some new functions for creating `Attributes`s directly,
without involving `MetaItem`s: `mk_attr_word`, `mk_attr_name_value_str`,
`mk_attr_nested_word`, and
`ExtCtxt::attr_{word,name_value_str,nested_word}`.

These new methods replace the old functions for creating `Attribute`s:
`mk_attr_inner`, `mk_attr_outer`, and `ExtCtxt::attribute`. Those
functions took `MetaItem`s as input, and relied on many other functions
that created `MetaItems`, which are also removed: `mk_name_value_item`,
`mk_list_item`, `mk_word_item`, `mk_nested_word_item`,
`{MetaItem,MetaItemKind,NestedMetaItem}::token_trees`,
`MetaItemKind::attr_args`, `MetaItemLit::{from_lit_kind,to_token}`,
`ExtCtxt::meta_word`.

Overall this cuts more than 100 lines of code and makes thing simpler.

compiler/rustc_builtin_macros/src/global_allocator.rs

@@ -115,9 +115,7 @@ impl AllocFnFactory<'_, '_> {
     }
 
     fn attrs(&self) -> AttrVec {
-        let special = sym::rustc_std_internal_symbol;
-        let special = self.cx.meta_word(self.span, special);
-        thin_vec![self.cx.attribute(special)]
+        thin_vec![self.cx.attr_word(sym::rustc_std_internal_symbol, self.span)]
     }
 
     fn arg_ty(