Improved svn repo searching for 'make update'

Use a simple recursive bash function to search for svn repos for the 'make
update' target thus including projects like clang-tools-extra.

Reviewers: bkramer, echristo


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173650 91177308-0d34-0410-b5e6-96231b3b80d8