[clang] [clang] Add -fcrash-diagnostics-tar for tarball of crash reproducer files (PR #198838)

[Jon Roelofs via cfe-commits]

jroelofs wrote:

> I think it'd be nice to turn this on by default and have it automatically dump a tarball in the same directory alongside the other crash reproducer files, but that should be a separate PR

That this new flag takes a path to the desired tar file means it doesn't compose well nor respect some of the existing facilities for determining where the crash reproducers are located, i.e. `-fcrash-diagnostics-dir` / `env CLANG_CRASH_DIAGNOSTICS_DIR=<path>`... and the latter is really important for collecting crash reproducers in CI systems.

https://github.com/llvm/llvm-project/pull/198838