Changed builld-llvm.py to use .json files

LLDB has many branches in a variety of repositories.
The build-script.py file is subtly different for each set.
This is unnecessary and causes merge headaches.

This patch makes build-llvm.py consult a directory full 
of .json files, each one of which matches a particular
branch using a regular expression.

This update to the patch introduces a FALLBACK file
whose contents take precedence if the current branch
could not be identified.  If the current branch could be
identified, FALLBACK is updated, allowing the user to
e.g. cut branches off of known branches and still have
the automatic checkout mechanism work.

It also documents all of this.

Differential revision: https://reviews.llvm.org/D30275

llvm-svn: 295922