sstables/processing_result_generator.hh: refine check for coroutine standard
sstables/processing_result_generator.hh: refine check for coroutine standard

We have a check for whether we can use standard coroutines (in namespace
std) or the technical specification (in std::experimental), but it doesn't
work since Clang doesn't report the correct standard version. Use a
compiler versionspecific check, inspired by Seastar's check.

This allows building with clang 14.

Closes #10603

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla