Work around a Clang bug (compilation error)
Clang rejects this standard-compliant code with the following error messages:
- default member initializer for 'count' needed within definition of enclosing class 'DUChainReferenceCounting' outside of member unctions
- <same as (1) but with 'intervals' in place of 'count'>
This change does not affect performance of DUChainReferenceCounting-sensitive benchmarks.