Let test-release.sh checkout subprojects directly into the target tree,

instead of using symlinks

Summary:
In the past I have run into several problems with the way
`test-release.sh` creates all the subproject directories as siblings,
and then uses symlinks to stitch them all together.  In some scenarios
this leads to clang not being able to find header files, etc.

This patch changes the script so it directly exports into the correct
target locations for each subproject.

Reviewers: hans

Subscribers: emaste, llvm-commits

Differential Revision: http://reviews.llvm.org/D16420

llvm-svn: 258436