The checkout configs included some old dated branches that are no longer
used. They are unlikely to work well anyway because the corresponding
Clang/LLVM branches moved forward independently. Remove those config entries.
Also re-order the 4.0 and 4.1 configs to be consistent.
This makes update_checkout into a true python module that we can add nosetests
for, readme, and also split up without issue into multiple small files.
I also added a small stub test directory and support for running tests for
update_checkout during validation-testing. So once we add some tests, everything
is setup correctly in terms of the swift build for testing purposes.
