Auto merge of #28642 - petrochenkov:name3, r=nrc

This PR removes random remaining `Ident`s outside of libsyntax and performs general cleanup
In particular, interfaces of `Name` and `Ident` are tidied up, `Name`s and `Ident`s being small `Copy` aggregates are always passed to functions by value, and `Ident`s are never used as keys in maps, because `Ident` comparisons are tricky.

Although this PR closes https://github.com/rust-lang/rust/issues/6993 there's still work related to it:
- `Name` can be made `NonZero` to compress numerous `Option<Name>`s and `Option<Ident>`s but it requires const unsafe functions.
- Implementation of `PartialEq` on `Ident` should be eliminated and replaced with explicit hygienic, non-hygienic or member-wise comparisons.
- Finally, large parts of AST can potentially be converted to `Name`s in the same way as HIR to clearly separate identifiers used in hygienic and non-hygienic contexts.

r? @nrc

src/libsyntax/parse/parser.rs

@@ -4658,7 +4658,7 @@ impl<'a> Parser<'a> {
             (fields, None)
         // Tuple-style struct definition with optional where-clause.
         } else if self.token == token::OpenDelim(token::Paren) {
-            let fields = try!(self.parse_tuple_struct_body(&class_name, &mut generics));
+            let fields = try!(self.parse_tuple_struct_body(class_name, &mut generics));
             (fields, Some(ast::DUMMY_NODE_ID))
         } else {
             let token_str = self.this_token_to_string();
@@ -4693,7 +4693,7 @@ impl<'a> Parser<'a> {
     }
 
     pub fn parse_tuple_struct_body(&mut self,
-                                   class_name: &ast::Ident,
+                                   class_name: ast::Ident,
                                    generics: &mut ast::Generics)
                                    -> PResult<Vec<StructField>> {
         // This is the case where we find `struct Foo<T>(T) where T: Copy;`
@@ -5723,10 +5723,10 @@ impl<'a> Parser<'a> {
                                                  Option<ast::Name>)>> {
         let ret = match self.token {
             token::Literal(token::Str_(s), suf) => {
-                (self.id_to_interned_str(s.ident()), ast::CookedStr, suf)
+                (self.id_to_interned_str(ast::Ident::with_empty_ctxt(s)), ast::CookedStr, suf)
             }
             token::Literal(token::StrRaw(s, n), suf) => {
-                (self.id_to_interned_str(s.ident()), ast::RawStr(n), suf)
+                (self.id_to_interned_str(ast::Ident::with_empty_ctxt(s)), ast::RawStr(n), suf)
             }
             _ => return Ok(None)
         };