rc3, forward port testing improvements, rebase pre-C11 fallbacks

[h-vetinari]

From #127; let's see how far back in clang we can go without the revert for C11.

[conda-forge-webservices]

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[h-vetinari]

OK, without the revert of llvm/llvm-project@21f73d5, libcxx 17 fails with clang 12 and older.

I've rebased the revert now, but that was only feasible because the conflicts were trivial. If the conflicts become complicated, or the added tests for new libcxx plus a very old clang fail and we cannot salvage it easily, then that's IMO just something to accept and move on.

Given that libcxx 17 only officially supports clang 15-17, We're already more than doubling the upstream support here (3 -> 7 clang versions).

CC @conda-forge/libcxx

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