This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvmorg-12-init-17869-g8e464dd76bef, the last commit before the
upstream release/12.x branch was created.
PR: 255570
MFC after: 6 weeks
master 2e10b7a39b9, the last commit before the llvmorg-12-init tag, from
which release/11.x was branched.
Note that for now, I rolled back all our local changes to make merging
easier, and I will reapply the still-relevant ones after updating to
11.0.0-rc1.
