Merge llvm-project release/18.x llvmorg-18.1.5-0-g617a15a9eac9

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project release/18.x llvmorg-18.1.5-0-g617a15a9eac9.

PR: 276104
Approved by: re (cperciva)
MFC after: 3 days

(cherry picked from commit 5678d1d98a348f315453555377ccb28821a2ffcd)
(cherry picked from commit fe72d8ecb98dd51a9faf62b1e14d4dde25d61e5b)