add check to allow symbolic save_idxs

[xtalax]

merge before https://github.com/SciML/ModelingToolkit.jl/pull/2052

[codecov[bot]]

Codecov Report

Merging #380 (3c862a1) into master (1228b9c) will decrease coverage by 0.32%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
- Coverage   57.61%   57.29%   -0.32%     
==========================================
  Files          44       44              
  Lines        3324     3325       +1     
==========================================
- Hits         1915     1905      -10     
- Misses       1409     1420      +11     

Impacted Files	Coverage Δ
src/solutions/solution_interface.jl	58.66% <100.00%> (+0.09%)	:arrow_up:
src/retcodes.jl	15.78% <0.00%> (-15.79%)	:arrow_down:
src/remake.jl	76.92% <0.00%> (-0.97%)	:arrow_down:
src/utils.jl	60.92% <0.00%> (-0.67%)	:arrow_down:

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