Merge llvm-project 12.0.1 rc2
This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvmorg-12.0.1-rc2-0-ge7dac564cd0e, a.k.a. 12.0.1 rc2.
PR: 255570
MFC after: 6 weeks
(cherry picked from commit 23408297fbf3089f0388a8873b02fa75ab3f5bb9)