Fix ICEs that involved quasi-quotation

* fixed get_tt for doc comments
* properly handle MatchNt in `quote`

Fixes #18763
Fixes #18775
auto merge of #18811 : pczarn/rust/issue-18763-ice, r=pnkfelix

Fix ICEs introduced in #17830

* fixed get_tt for doc comments
* properly handle MatchNt in `quote`

Fixes #18763
Fixes #18775