llvm_tools: refactor to support multiple SHA sources

This refactors code a bit and tweaks our CLI so we can more easily
support various sources of 'interesting' SHAs. The intent going forward
is to have one program invocation per SHA source (e.g., one Android, one
CrOS, etc).

BUG=None
TEST=Ran the script

Change-Id: Ic3c682985ca688aa0e0ac38902d88f8b6dd2a223
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2173674
Reviewed-by: Tiancong Wang <tcwang@google.com>
Tested-by: George Burgess <gbiv@chromium.org>
2 files changed