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.

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

llvm-svn: 295897