[test][OpenMP] Avoid writing to a potentially write-protected dir (#94931)

The test clang/test/OpenMP/error_unsupport_feature.c don't check the
output written to the current directory. The current directory may be
write protected e.g. in a sandboxed environment.

This patch replace the -emit-llvm option with -emit-llvm-only as it
don't care about the outputed llvm IR.