Suggested changes to Variables.qll
Suggested changes to Variables.qll

- Remove `abstract` predicates from public API.
- Cache core computations.
- Redefine `VariableScope::get[A]Variable` to only include variables declared
  directly in the scope.

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