Implement token-based handling of attributes during expansion

This PR modifies the macro expansion infrastructure to handle attributes
in a fully token-based manner. As a result:

* Derives macros no longer lose spans when their input is modified
  by eager cfg-expansion. This is accomplished by performing eager
  cfg-expansion on the token stream that we pass to the derive
  proc-macro
* Inner attributes now preserve spans in all cases, including when we
  have multiple inner attributes in a row.

This is accomplished through the following changes:

* New structs `AttrAnnotatedTokenStream` and `AttrAnnotatedTokenTree` are introduced.
  These are very similar to a normal `TokenTree`, but they also track
  the position of attributes and attribute targets within the stream.
  They are built when we collect tokens during parsing.
  An `AttrAnnotatedTokenStream` is converted to a regular `TokenStream` when
  we invoke a macro.
* Token capturing and `LazyTokenStream` are modified to work with
  `AttrAnnotatedTokenStream`. A new `ReplaceRange` type is introduced, which
  is created during the parsing of a nested AST node to make the 'outer'
  AST node aware of the attributes and attribute target stored deeper in the token stream.
* When we need to perform eager cfg-expansion (either due to `#[derive]` or `#[cfg_eval]`),
we tokenize and reparse our target, capturing additional information about the locations of
`#[cfg]` and `#[cfg_attr]` attributes at any depth within the target.
This is a performance optimization, allowing us to perform less work
in the typical case where captured tokens never have eager cfg-expansion run.

compiler/rustc_parse/src/parser/nonterminal.rs

@@ -153,9 +153,7 @@ impl<'a> Parser<'a> {
             NonterminalKind::Path => token::NtPath(
                 self.collect_tokens_no_attrs(|this| this.parse_path(PathStyle::Type))?,
             ),
-            NonterminalKind::Meta => {
-                token::NtMeta(P(self.collect_tokens_no_attrs(|this| this.parse_attr_item(false))?))
-            }
+            NonterminalKind::Meta => token::NtMeta(P(self.parse_attr_item(true)?)),
             NonterminalKind::TT => token::NtTT(self.parse_token_tree()),
             NonterminalKind::Vis => token::NtVis(
                 self.collect_tokens_no_attrs(|this| this.parse_visibility(FollowedByType::Yes))?,