Write test for CWG1772/CWG1762/CWG1779, mark them 'done', and update

cxx_dr_status.html

I noticed that these two DRs are currently working correctly, so I
added a pair of lit tests that check the AST (which is most useful for
CWG1779, since 'dependent' is really only observable in an ast dump) to
make sure __func__ works correctly in dependent cases, and in lambda
operator().

Also noticed that CWG1762, mostly an 'example' change, works correctly,
so added a test so that it gets marked 'done' as well.

Additionally, I regenerated cxx_dr_status.html, updating it for Clang
13's release, based on the cwg_status.html from August 12, 2021.

Differential Revision: https://reviews.llvm.org/D109956