Merge llvm-project main llvmorg-18-init-16003-gfc5f51cf5af4

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project main llvmorg-18-init-16003-gfc5f51cf5af4.

PR: 276104
MFC after: 1 month

Merge llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and openmp
master 2e10b7a39b9, the last commit before the llvmorg-12-init tag, from
which release/11.x was branched.

Note that for now, I rolled back all our local changes to make merging
easier, and I will reapply the still-relevant ones after updating to
11.0.0-rc1.

Merge ^/vendor/lvm-project/master up to its last change (upstream commit
e26a78e70), and resolve conflicts.