Roll back claims that we implement P0960R3 and P1975R0

We thought we had implemented these papers appropriately but have since
discovered significant issues. See discussion of the issues at:
https://github.com/llvm/llvm-project/issues/61145

The work already done on these papers is remaining in tree for the
moment while people investigate whether the issues can be fixed
forward in main. The status page is being updated so the status is
clear to users of the upcoming Clang 16 release.