[lldb/Reproducers] Override capture with LLDB_CAPTURE_REPRODUCER env var

Make it possible to override reproducer capture with the
LLDB_CAPTURE_REPRODUCER environment variable.

The goal of this change is twofold.

(1) I want to be able to enable capturing reproducers during regular
    test runs, both locally and on the bots. To do so I need a way to
    force capture. I cannot do this through the Python API, because
    reproducer capture must be enabled *before* the debugger
    initialized, which happens automatically when doing `import lldb`.

(2) I want to provide an escape hatch for when reproducers are enabled
    by default. Downstream we have reproducer capture enabled by default
    in the driver.

This patch solves both problems by overriding the reproducer mode based
on the environment variable. Acceptable values are 0/1 and ON/OFF.