Merge llvm-project 12.0.0 release
This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvmorg-12.0.0-0-gd28af7c654d8, a.k.a. 12.0.0 release.
PR: 255570
MFC after: 6 weeks
(cherry picked from commit d409305fa3838fb39b38c26fc085fb729b8766d5)