Enable use of unquote in defguard(p)

[alisinabh]

Currently, it is not possible to define a guard in Elixir using a dynamic name at compile time like it it possible with def and defmacro.

As an example the following code fails to compile

defmodule MyModule do
  @states [:a, :b, :c]

  for state <- @states do
    defguard unquote(String.to_atom("is_#{state}"))(state) when state == unquote(state)
  end
end

== Compilation error in file text.ex ==
** (ArgumentError) invalid syntax in defguard unquote(String.to_atom("is_#{state}"))(state)
    (elixir 1.17.0) lib/kernel.ex:5789: Kernel.define_guard/3
    (elixir 1.17.0) expanding macro: Kernel.defguard/1
    text.ex:5: MyModule (module)

This ensures that an unquote syntax is also accepted in defguard and defguardp + tests.

[josevalim]

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