ci: standardize custom libc++ usage in MSAN jobs #27485

Use -isystem & -nostd* flags, which is the preferred way to use a custom libc++ (ours is libc++ build with MSAN) with Clang, as opposed to our current ad-hoc flags.

See: https://releases.llvm.org/16.0.0/projects/libcxx/docs/UsingLibcxx.html#using-a-custom-built-libc for more info:

Most compilers provide a way to disable the default behavior for finding the standard library and to override it with custom paths. With Clang, this can be done with:

clang++ -nostdinc++ -nostdlib++             \
          -isystem <install>/include/c++/v1 \
          -L <install>/lib                  \
          -Wl,-rpath,<install>/lib          \
          -lc++                             \
          test.cpp

The native_fuzz_with_msan job is still broken (on master), which is related to using system compiler-rt, see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1005341 for more info. I think standardising how we use our libc++ is a first step towards fixing. Although this may ultimately require building a clang and compiler-rt ourselves.

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We can likely do something similar for our Darwin depends build, once we're using a newer version of LLVM/Clang, which might make the flag usage and configuration in there slightly clearer.

We can likely do something similar for our Darwin depends build

See #27493

Rebased past #27448, and taking out of draft.

Going to open a new PR with more extensive changes.