It was deprecated in LLVM 15, turned into a compile-time error in LLVM 16:
```bash
In file included from /usr/lib/llvm-16/bin/../include/c++/v1/cassert:19:
/usr/lib/llvm-16/bin/../include/c++/v1/__assert:22:5: error: "Defining _LIBCPP_DEBUG is not supported anymore.
Please use _LIBCPP_ENABLE_DEBUG_MODE instead."
^
1 error generated.
```
and has been removed entirely in LLVM 17 (main),
ff573a42cd.
Building libc++ in debug mode, will also automatically set
`_LIBCPP_ENABLE_DEBUG_MODE` (the new define), so adding it to depends
doesn't seem useful, and would just result in redefinition errors.
I'm wondering if as a followup, we could enable a DEBUG build of libc++
in our MSAN CI job?
Somewhat related to https://github.com/google/oss-fuzz/pull/9828, where
it looks like we'll have to sort out getting a DEBUG build of LLVM.
