[llvm] [libcxx] [libc++][hardening] Rework macros for enabling the hardening mode. (PR #70575)

[Louis Dionne via llvm-commits]

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@@ -0,0 +1 @@
+set(LIBCXX_HARDENING_MODE "fast" CACHE STRING "")
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ldionne wrote:

You could consider naming these `Generic-hardening-mode-debug.cmake` (or just `Generic-hardening-debug.cmake`) & al so that they show up side by side when browsing files. Not necessary, but might be nice.

https://github.com/llvm/llvm-project/pull/70575