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_expand/src/build.rs

@@ -616,11 +616,23 @@ impl<'a> ExtCtxt<'a> {
         self.item(span, name, AttrVec::new(), ast::ItemKind::Const(def, ty, Some(expr)))
     }
 
-    pub fn attribute(&self, mi: ast::MetaItem) -> ast::Attribute {
-        attr::mk_attr_outer(&self.sess.parse_sess.attr_id_generator, mi)
+    // Builds `#[name]`.
+    pub fn attr_word(&self, name: Symbol, span: Span) -> ast::Attribute {
+        let g = &self.sess.parse_sess.attr_id_generator;
+        attr::mk_attr_word(g, ast::AttrStyle::Outer, name, span)
     }
 
-    pub fn meta_word(&self, sp: Span, w: Symbol) -> ast::MetaItem {
-        attr::mk_word_item(Ident::new(w, sp))
+    // Builds `#[name = val]`.
+    //
+    // Note: `span` is used for both the identifer and the value.
+    pub fn attr_name_value_str(&self, name: Symbol, val: Symbol, span: Span) -> ast::Attribute {
+        let g = &self.sess.parse_sess.attr_id_generator;
+        attr::mk_attr_name_value_str(g, ast::AttrStyle::Outer, name, val, span)
+    }
+
+    // Builds `#[outer(inner)]`.
+    pub fn attr_nested_word(&self, outer: Symbol, inner: Symbol, span: Span) -> ast::Attribute {
+        let g = &self.sess.parse_sess.attr_id_generator;
+        attr::mk_attr_nested_word(g, ast::AttrStyle::Outer, outer, inner, span)
     }
 }