Make DUChainReferenceCounting::instance() noexcept

This change does not affect performance of
DUChainReferenceCounting-sensitive benchmarks.
1 job for !207 with referencecounting-clang-workaround in 36 minutes and 51 seconds (queued for 13 minutes and 23 seconds)
detached
Status Job ID Name Coverage
  Build
passed #83233
linux kf5-qt5 qt5.15

00:36:51

46.0%