[clang] Implement setting crash_diagnostics_dir through env variable

This implements setting the equivalent of `-fcrash-diagnostics-dir`
through the environment variable `CLANG_CRASH_DIAGNOSTICS_DIR`.
If present, the flag still takes precedence.

This helps integration with test frameworks and pipelines.

With this feature, we change the libcxx bootstrapping build
pipeline to produce clang crash reproducers as artifacts.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Differential Revision: https://reviews.llvm.org/D133082