Merge bitcoin/bitcoin#27447: depends: Remove `_LIBCPP_DEBUG` from depends DEBUG mode

bc4fd49d09dec3791b0acd4ada285b2287361d14 depends: add _LIBCPP_ENABLE_ASSERTIONS to DEBUG mode (fanquake)
cf266b2270081f05a277ba683f00de90a741112f depends: Remove _LIBCPP_DEBUG from depends DEBUG mode (fanquake)

Pull request description:

  It was deprecated in LLVM 15, turned into 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): https://github.com/llvm/llvm-project/commit/ff573a42cd1f1d05508f165dc3e645a0ec17edb5.

  [Building libc++ in debug mode](https://releases.llvm.org/16.0.0/projects/libcxx/docs/DesignDocs/DebugMode.html), 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? i.e https://github.com/fanquake/bitcoin/tree/msan_with_enable_debug_mode.

  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, and can drop the commentary about re-enabling DEBUG=1.

ACKs for top commit:
  MarcoFalke:
    lgtm Approach ACK bc4fd49d09dec3791b0acd4ada285b2287361d14

Tree-SHA512: 9c0f48fc428278fbf34fbb8f81e761e232506d7ab28e971cb9a9b9a81d549b4d8bbe51e2f7608d56e489428679231da5b7431443849b238a8a993ad241740282

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