Declaring a variable via `out` breaks when the type of the variable changes

[straight-shoota]

When a variable declared as an out parameter of lib call is later assigned a different type, we get memory corruption:

lib LibC
  fun modf(value : Float64, iptr : Float64*) : Float64
end
 
LibC.modf(32.14, out foo)
foo = {foo, foo}
foo # => {0.0, 0.0}

If we change the assignment to a different variable than foo, everything works as expected (value is {32.0, 32.0}).

The behaviour appears in both the compiler and interpreter, so I presume it's a semantic error.

This error came up in #16555 (comment).

I suppose a simple solution would be to disallow changing the type of an out var: out should act like a type declaration and freeze the type of the variable, making it impossible to produce any confusion.

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[ysbaddaden]

It looks like the variable shadowing isn't working when the variable has been declared as out.