Provide "semi-stable" and historical versions in the binary repo

[kerberizer]

The newly committed GLSL/TGSI on-disk cache for radeonsi relies on the LLVM (and Mesa) build times to validate the cache files. With our binary repo pushing out new builds every six hours and the cache going to be enabled by default, this likely means that the shader cache will be getting stale way too quickly.

It might be therefore beneficial if we also provide a "semi-stable" repo(s), which will get updated less frequently, e.g. once a week, once a month, etc. Related to this is the problem with broken builds that sometimes happen (e.g. see the comment on AUR from 2016-11-07 14:45). If we keep "historical versions" of the built packages, this would help the users revert to a last known good state without too much hassle.

A crude first plan is to have the following additional repos:

• llvm-svn-monthly: gets updated on the 1st of each month;
• llvm-svn-weekly: gets updated each Friday at around 0:00 (I guess lots of people do their gaming on the weekends)
• llvm-svn-YYYYMMDD: gets "branched" every day and stays for a number of days (depending on the storage availability this might be 10, 30 days or even more)