llvm18 (especially clang) brings some breaking changes, which causes
build failure in llvm-project.
This patch brings commits from llvm-project upstream which fixes build
failure for llvm-project using devel/llvm18.
b9935bb02a50 and 966d564e43e6 fix compiler error by invalid assembly
displacement on x86_86 systems.
a4d1d5f5b54b2f93d7290588734f59ff24fc515c and
a7ac0dd624962de1ccb55f1ed1357f548477f593 replaces variable length arrays
with a custom class `SimpleVLA`.
There are other parts of code in openmp, so `-Wno-vla-cxx-extension` is
added to lib/libomp/Makefile.