Remove `Lexer`'s dependency on `Parser`.

Lexing precedes parsing, as you'd expect: `Lexer` creates a
`TokenStream` and `Parser` then parses that `TokenStream`.

But, in a horrendous violation of layering abstractions and common
sense, `Lexer` depends on `Parser`! The `Lexer::unclosed_delim_err`
method does some error recovery that relies on creating a `Parser` to do
some post-processing of the `TokenStream` that the `Lexer` just created.

This commit just removes `unclosed_delim_err`. This change removes
`Lexer`'s dependency on `Parser`, and also means that `lex_token_tree`'s
return value can have a more typical form.

The cost is slightly worse error messages in two obscure cases, as shown
in these tests:
- tests/ui/parser/brace-in-let-chain.rs: there is slightly less
  explanation in this case involving an extra `{`.
- tests/ui/parser/diff-markers/unclosed-delims{,-in-macro}.rs: the diff
  marker detection is no longer supported (because that detection is
  implemented in the parser).

In my opinion this cost is outweighed by the magnitude of the code
cleanup.

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