Revert "De-share ast::Ty"

This reverts commit 47eca2113c5c55a4ffbb8c11a38c8ca7a79a1d72.
auto merge of #7678 : brson/rust/deshareastty, r=brson

This reverts commit 47eca2113c5c55a4ffbb8c11a38c8ca7a79a1d72.