The return-empty-string-non-mangled.patch apparently landed in r249437
upstream, so we don't need to carry around our own version. The patch
is still nominally used for building clang on Mac, so I have not removed
it from version control.
We need to rebuild clang with libc++ to get compatible headers for cross
builds. libc++abi is a dependency of libc++, as the build instructions
says [0].
[0] http://libcxx.llvm.org/docs/BuildingLibcxx.html
Ideally, we'd use the tarballs from
http://llvm.org/releases/download.html but I didn't feel like modifying
the script more than I already did to make it work at all (bug 1262735).
The new tarball for Linux was built on
https://tools.taskcluster.net/task-inspector/#LCUn8aEgTBeRJ11a3qTlDQ/0
The new tarball for Mac was built on a loaner, after installing cmake
and ninja, as well as building ld64 127.2 from source because the
installed version would assert while building clang. The latter required
manually adding some missing headers to /usr/include. TSAN was also
disabled because it requires APIs that are not available on the OSX
version on the build slaves (e.g. pthread_introspection_hook_install).
Building clang also required using a mac clang from tooltool, the system
one lacking support for atomics.
Both point to the same path for cc and cxx, but not for gcc_dir, which
makes no sense. That's the only significant difference between both, so
just merge them both, and use the merged file in the taskcluster build
script.