Move libunwind out of contrib/llvm/projects.

[jhb]

Summary

Move LLVM's libunwind to its own contrib/ directory similar to other
runtime libraries like libc++ and libcxxrt.

Test Plan

• make universe

I will take a shot at updating it to the latest imported version as the
next step after this.

[dim]

LGTM although this rather disrupts the work I have been doing in D19529. If you want to take over updating libunwind I'll be glad though. :-)

[jhb]

• Update after dim@ merged in libunwind changes.

[emaste]

So this is just svn mv + path change in lib/libgcc_eh/Makefile.inc? Fine with me.

[jhb]

In D19534#418446, @emaste wrote:

So this is just svn mv + path change in lib/libgcc_eh/Makefile.inc? Fine with me.

Yes. I did take the opportunity to purge some mergeinfo from the directory as well. All that it has in the new location is mergeinfo from vendor/llvm/dist/projects/libunwind and vendor/libunwind-llvm/dist