rustdoc: show struct field docs when inlined

Some minor changes to the compiler to expose this information. Very
inconvenient since struct fields aren't an item. Adds (yet another) table to
metadata.

Closes #15739
rustdoc: remove extraneous .move_iter().collect()s

The impl of Clean for Vec obsoleted these long, long ago.
rustc: encode is_sugared_doc on ast::Attribute
auto merge of #16025 : cmr/rust/plugin-fields, r=alexcrichton

Some minor changes to the compiler to expose this information. Very
inconvenient since struct fields aren't an item.