C++: Make `Declaration` not abstract
C++: Make `Declaration` not abstract

It looks like this change will stop `SignedOverflowCheck.ql` from
needlessly re-evaluating several cached stages.

github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql