This CL introduces the TargetLowering::AutoBundle type, which allows
RAII-style bundle emission. As part of the CL, all of the uses of
TargetLowering::_bundle_lock(), and TargetLowering::_bundle_unlock(),
were replaced with uses of the newly introduced type.

BUG=
R=sehr@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1585843007 .