Support Sample with multiple implicit clocks

[YingboMa]

Fix #2731

[baggepinnen]

I added a partial test, but it fails with

julia> sol = solve(prob, Tsit5(); kwargshandle = KeywordArgSilent)
ERROR: UndefVarError: `r` not defined
Stacktrace:
  [1] (::Tuple{})(arg#5964805160111424296::Any, arg#16064127567989806372::Any, arg#16674255279130770513::Any, t::Any)
    @ ModelingToolkit ~/.julia/packages/SymbolicUtils/c0xQb/src/code.jl:375
  [2] macro expansion
    @ ~/.julia/dev/ModelingToolkit/src/systems/clock_inference.jl:473 [inlined]
  [3] macro expansion
    @ ~/.julia/packages/RuntimeGeneratedFunctions/M9ZX8/src/RuntimeGeneratedFunctions.jl:163 [inlined]
  [4] macro expansion
    @ ./none:0 [inlined]
  [5] generated_callfunc
    @ ./none:0 [inlined]
  [6] (::RuntimeGeneratedFunctions.RuntimeGeneratedFunction{…})(::OrdinaryDiffEq.ODEIntegrator{…}, ::SavedValues{…})
    @ RuntimeGeneratedFunctions ~/.julia/packages/RuntimeGeneratedFunctions/M9ZX8/src/RuntimeGeneratedFunctions.jl:150
  [7] DiscreteSaveAffect
    @ ~/.julia/dev/ModelingToolkit/src/systems/diffeqs/abstractodesystem.jl:1074 [inlined]

[YingboMa]

@AayushSabharwal is the ModelingToolkitStandardLibrary test failure unrelated?