Merge llvm-project main llvmorg-12-init-17869-g8e464dd76bef
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