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_ast/src/util/literal.rs

@@ -206,15 +206,6 @@ impl MetaItemLit {
         token::Lit::from_token(token)
             .and_then(|token_lit| MetaItemLit::from_token_lit(token_lit, token.span).ok())
     }
-
-    /// Losslessly convert a meta item literal into a token.
-    pub fn to_token(&self) -> Token {
-        let kind = match self.token_lit.kind {
-            token::Bool => token::Ident(self.token_lit.symbol, false),
-            _ => token::Literal(self.token_lit),
-        };
-        Token::new(kind, self.span)
-    }
 }
 
 fn strip_underscores(symbol: Symbol) -> Symbol {