Improve -Z crate-attr diagnostics

- Show the `#![ ... ]` in the span (to make it clear that it should not
  be included in the CLI argument)
- Show more detailed errors when the crate has valid token trees but
  invalid syntax.
  Previously, `crate-attr=feature(foo),feature(bar)` would just say
  "invalid crate attribute" and point at the comma. Now, it explicitly
  says that the comma was unexpected, which is useful when using
  `--error-format=short`. It also fixes the column to show the correct
  span.
- Recover from parse errors. Previously we would abort immediately on
  syntax errors; now we go on to try and type-check the rest of the
  crate.

The new diagnostic code also happens to be slightly shorter.

compiler/rustc_ast/src/attr/mod.rs

@@ -627,7 +627,7 @@ pub fn mk_doc_comment(
     Attribute { kind: AttrKind::DocComment(comment_kind, data), id: g.mk_attr_id(), style, span }
 }
 
-pub fn mk_attr(
+fn mk_attr(
     g: &AttrIdGenerator,
     style: AttrStyle,
     unsafety: Safety,