depends: Add missing -D_LIBCPP_DEBUG=1 to debug flags #21869

Commands that can be used for testing:

$ cat 1.cpp 
#include <vector>

int main() {
  std::vector<int> foo;
  foo.begin() + 7;
}

clang++ -stdlib=libc++ -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC -D_LIBCPP_DEBUG=1 -Wall 1.cpp -o exe && ./exe  
g++                    -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC -D_LIBCPP_DEBUG=1 -Wall 1.cpp -o exe && ./exe 

Suggested by @guidovranken in https://github.com/google/oss-fuzz/issues/5713#issuecomment-833034501. Thanks!

Concept ACK. https://libcxx.llvm.org/docs/DesignDocs/DebugMode.html.

cr ACK fa9249aaccc3ef7a0a91a822e1cb666c4c9716ec: patch looks correct

ACK fa9249aaccc3ef7a0a91a822e1cb666c4c9716ec - was going to suggest adding this to the macOS CPP flags as well, however it seems doing that is less straight forward. Could be looked at by someone in a followup.