Avoid recursive store for String

[vchuravy]

@wsmoses unsure how to test this:

I did hit:

Trixi.TreeMesh{2, Trixi.SerialTree{2, Float64}, Float64}
Trixi.SerialTree{2, Float64}
String
ERROR: AssertionError: fieldcount(Ty) != 0
Stacktrace:
  [1] create_recursive_stores(B::LLVM.IRBuilder, Ty::DataType, prev::LLVM.Value)
    @ Enzyme.Compiler ~/src/Enzyme/src/compiler.jl:614
  [2] create_recursive_stores(B::LLVM.IRBuilder, Ty::DataType, prev::LLVM.Value)
    @ Enzyme.Compiler ~/src/Enzyme/src/compiler.jl:658
  [3] shadow_alloc_rewrite(V::Ptr{…}, gutils::Ptr{…}, Orig::Ptr{…}, idx::UInt64, prev::Ptr{…}, used::UInt8)
   `` @ Enzyme.Compiler ~/src/Enzyme/src/compiler.jl:703

[github-actions]

Benchmark Results

	main	75f18b7...	main/75f18b724a7b1f...
basics/overhead	5.26 ± 0.01 ns	5.87 ± 0.011 ns	0.896
time_to_load	1.11 ± 0.0032 s	1.11 ± 0.0019 s	0.997

Benchmark Plots

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Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).