[lldb] Avoid using any shell when calling xcrun.

When we run `xcrun` we don't have any user input in our command so relying on
the user's default shell doesn't make a lot of sense. If the user has set the
system shell to a something that isn't supported yet (dash, ash) then we would
run into the problem that we don't know how to escape our command string.

This patch just avoids using any shell at all as xcrun is always at the same
path.

Reviewed By: aprantl, JDevlieghere, kastiglione

Differential Revision: https://reviews.llvm.org/D104653