C++: Avoid `extends Operation` in LeapYear.qll
C++: Avoid `extends Operation` in LeapYear.qll

The `Operation` class is abstract, and extending it caused cached stages
to be recomputed all the way down to the AST. This meant that the leap
year queries evaluated their own copy of SSA and data flow.

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