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
Not connected to the main build yet, as there is still the issue of the GNU omp.h header conflicting with the LLVM one. (That is, if MK_GCC is enabled.) PR: 236062 MFC after: 1 month X-MFC-With: r344779