Fix modifying user-expressions in macros

[odow]

Closes #3638

Looks like this was caused by the new MA.rewrite( ; move_factors_into_sums = false) code path in #3106

julia> JuMP._MA.rewrite(:(e))
(Symbol("##308"), :(var"##308" = begin
          #= /Users/oscar/.julia/packages/MutableArithmetics/NIXlP/src/rewrite.jl:325 =#
          let
              #= /Users/oscar/.julia/packages/MutableArithmetics/NIXlP/src/rewrite.jl:326 =#
              begin
                  var"##310" = MutableArithmetics.Zero()
                  var"##309" = (MutableArithmetics.operate!!)(MutableArithmetics.add_mul, var"##310", $(Expr(:escape, :e)))
              end
              #= /Users/oscar/.julia/packages/MutableArithmetics/NIXlP/src/rewrite.jl:327 =#
              var"##309"
          end
      end))

julia> JuMP._MA.rewrite(:(e); move_factors_into_sums = false)
(Symbol("##311"), :(var"##311" = begin
          #= /Users/oscar/.julia/packages/MutableArithmetics/NIXlP/src/rewrite.jl:325 =#
          let
              #= /Users/oscar/.julia/packages/MutableArithmetics/NIXlP/src/rewrite.jl:326 =#
              begin
                  #= /Users/oscar/.julia/packages/MutableArithmetics/NIXlP/src/rewrite.jl:321 =#
              end
              #= /Users/oscar/.julia/packages/MutableArithmetics/NIXlP/src/rewrite.jl:327 =#
              $(Expr(:escape, :e))
          end
      end))

Note how the latter does not create a copy of the expression.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f5f07b5) 98.27% compared to head (a493931) 98.27%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3639   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files          43       43           
  Lines        5636     5639    +3     
=======================================
+ Hits         5539     5542    +3     
  Misses         97       97           

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