Revert "De-share ast::Ty"

This reverts commit 47eca2113c5c55a4ffbb8c11a38c8ca7a79a1d72.
auto merge of #8072 : graydon/rust/syntax-deshare, r=graydon

This reverts commit 47eca2113c5c55a4ffbb8c11a38c8ca7a79a1d72.

Rebased from @Aatch's previous PR #7644, attempting to land.