Merge llvm-project release/18.x llvmorg-18.1.0-rc2-0-gc6c86965d967
This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project release/18.x llvmorg-18.1.0-rc2-0-gc6c86965d967.
PR: 276104
MFC after: 1 month
(cherry picked from commit b3edf4467982447620505a28fc82e38a414c07dc)