Generated functions no longer re-tracable with Symbolics

[baggepinnen]

A recent change (maybe PR #536 @YingboMa ?) causes functions produced by build_function to fail when called with symbolic variables:

julia> @variables x
1-element Vector{Num}:
 x

julia> f = build_function([sin(x)], [x], expression=Val{false});

julia> f[1]([1])
1-element Vector{Float64}:
 0.8414709848078965

julia> f[1]([x])
ERROR: StackOverflowError:
Stacktrace:
 [1] sin(x::Num) (repeats 79984 times)
   @ NaNMath ~/.julia/packages/NaNMath/ceWIc/src/NaNMath.jl:11

This is a problem when generated functions are used to build, e.g., cost functions for optimization problems, after which symbolic tracing is used to find sparsity patterns or generate jacobian functions etc.