allow local increases of libc++ hardening level

Restores the behaviour of setting `_DEFAULT` in our site config (as is [recommended](https://libcxx.llvm.org/Hardening.html#notes-for-vendors) for "vendors"), allowing users to increase the libc++ hardening level of their builds using certain flags.

This behaviour is required for our continuous migration towards higher hardening levels, as one-off hardening runs are otherwise significantly more difficult.

This commit does not change the behaviour of where hardening is enabled, meaning that it should have no effect on build targets in general (though some targets that are broken in trunk after enabling hardening may not be marked as such yet).

Refer to the ticket for further information.
commit_hash:ba4f58ff13f9c21ac145d286f77308b38f10b89e

0 files changed, 0 insertions, 0 deletions