[JuliaLowering] Fix use-and-def analysis for `[function_decl]` (#61051)
This form is an implicit assignment to its argument, so the use-and-def
analysis needs to take this into account. Otherwise it's both a
correctness and optimization problem.
Resolves https://github.com/JuliaLang/JuliaLowering.jl/issues/147.
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Co-authored-by: Em Chu <61633163+mlechu@users.noreply.github.com>