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.
Differential revision: https://reviews.llvm.org/D30275
llvm-svn: 295897