…nt-expr, r=petrochenkov

Fix duplicated attributes on nonterminal expressions

This PR fixes a long-standing bug (rust-lang#86055) whereby expression attributes can be duplicated when expanded through declarative macros.

First, consider how items are parsed in declarative macros:
```
Items:
- parse_nonterminal
  - parse_item(ForceCollect::Yes)
    - parse_item_
      - attrs = parse_outer_attributes
      - parse_item_common(attrs)
        - maybe_whole!
        - collect_tokens_trailing_token
```
The important thing is that the parsing of outer attributes is outside token collection, so the item's tokens don't include the attributes. This is how it's supposed to be.

Now consider how expression are parsed in declarative macros:
```
Exprs:
- parse_nonterminal
  - parse_expr_force_collect
    - collect_tokens_no_attrs
      - collect_tokens_trailing_token
        - parse_expr
          - parse_expr_res(None)
            - parse_expr_assoc_with
              - parse_expr_prefix
                - parse_or_use_outer_attributes
                - parse_expr_dot_or_call
```
The important thing is that the parsing of outer attributes is inside token collection, so the the expr's tokens do include the attributes, i.e. in `AttributesData::tokens`.

This PR fixes the bug by rearranging expression parsing to that outer attribute parsing happens outside of token collection. This requires a number of small refactorings because expression parsing is somewhat complicated. While doing so the PR makes the code a bit cleaner and simpler, by eliminating `parse_or_use_outer_attributes` and `Option<AttrWrapper>` arguments (in favour of the simpler `parse_outer_attributes` and `AttrWrapper` arguments), and simplifying `LhsExpr`.

r? `@petrochenkov`