export.sh: Fetch sources from GitHub instead of SVN
Reviewers: hansw, jdoerfert
Subscribers: sylvestre.ledru, mgorny, hans, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D70460
(cherry picked from commit edf6717d8d30034da932b95350898e03c90a5082)