test-release.sh: Add ability to do a test build using the trunk or branches.

Summary:
Adds '--svn-path BRANCH' that causes the script to export the specified path
from each project. Otherwise the tag specified by -release, -rc, etc. will be
used. The version portion of the package name will be 'test-$path' (any forward
slashes in the branch name are replaced with underscores), for example:
  -svn-path trunk => clang+llvm-test-trunk-mips-linux-gnu.tar.xz
  -svn-path branches/release_35 => clang+llvm-test-branches_release_35-mips-linux-gnu.tar.xz

This is primarily useful for bringing new release packages up to standard
without needing to create and maintain a tag for the purpose.

Reviewers: tstellarAMD, hans

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6563

llvm-svn: 242518