Fix leaks caused by self-referential parameter constraints

[abadams]

This should fix memory leaks caused by Parameters that have constraints that refer back to the same Parameter.

Note that it doesn't fix leaks caused by Parameter A having a constraint that refers to Parameter B which has a constraint that refers back to Parameter A.

[steven-johnson]

Does this allow us to re-enable leak detection on the bots, or is there more (known) work to be done?

[abadams]

There's more work to be done.

[steven-johnson]

/home/halidenightly/build_bot/worker/halide-testbranch-main-llvm17-x86-64-linux-make/halide-source/src/Parameter.cpp: In function ‘Halide::Expr Halide::Internal::remove_self_references(const Halide::Internal::Parameter&, const Halide::Expr&)’:
/home/halidenightly/build_bot/worker/halide-testbranch-main-llvm17-x86-64-linux-make/halide-source/src/Parameter.cpp:206:14: error: ‘virtual Halide::Expr Halide::Internal::remove_self_references(const Halide::Internal::Parameter&, const Halide::Expr&)::RemoveSelfReferences::visit(const Halide::Internal::Variable*)’ can be marked override [-Werror=suggest-override]
  206 |         Expr visit(const Variable *var) {
      |              ^~~~~
/home/halidenightly/build_bot/worker/halide-testbranch-main-llvm17-x86-64-linux-make/halide-source/src/Parameter.cpp: In function ‘Halide::Expr Halide::Internal::restore_self_references(const Halide::Internal::Parameter&, const Halide::Expr&)’:
/home/halidenightly/build_bot/worker/halide-testbranch-main-llvm17-x86-64-linux-make/halide-source/src/Parameter.cpp:229:14: error: ‘virtual Halide::Expr Halide::Internal::restore_self_references(const Halide::Internal::Parameter&, const Halide::Expr&)::RestoreSelfReferences::visit(const Halide::Internal::Variable*)’ can be marked override [-Werror=suggest-override]
  229 |         Expr visit(const Variable *var) {
      |              ^~~~~

[abadams]

Failures unrelated