[JuliaLowering] Fix placeholders in parameters and decls, work around hasmethod

[mlechu]

Moved from c42f/JuliaLowering.jl#99

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• Fix placeholders in positional and keyword arguments.

  • Placeholder positional arguments sometimes need to be read from due to the
    way we desugar keyword functions; give them an internal name. (fix https://github.com/c42f/JuliaLowering.jl/issue/55, port Allow underscore (unused) args in presence of kwargs #58803)
  • Positional arguments may contain duplicate placeholders
  • Keywords may be placeholders (which is strange) but may not be duplicates
  • kws... may use a placeholder (fix https://github.com/c42f/JuliaLowering.jl/issue/49)

• Allow placeholders in decls: local/global _, local/global _::T. This
  can be produced by destructuring assignments. I've implemented this to
  always drop the type (see Assignment to _ sets the binding type of _ #57497),
  but this may not be what we want.

• Work around hasmethod always returning false for world=typemax(UInt) (some
  discussion here). I'm not sure we should be using hasmethod in the first place.

• Comment out our @atomic implementation. We need to implement this for all
  forms that the normal @atomic takes, otherwise it hijacks the expansion and
  expects a simple form.