std.builtin.Type.Pointer.Attributes: "of" helper function

[dotcarmen]

#23733 introduced a new type std.builtin.Type.Pointer.Attributes for usage with the new @Pointer builtin. however, there's not yet a convenient way to retrieve the information in this struct from a std.builtin.Type.Pointer.

this PR introduces a new of function to retrieve the pointer attributes from a std.builtin.Type.Pointer. this makes dynamic construction of pointer types simpler and mirrors usage of std.Alignment.of

[dotcarmen]

a sensible alternative would be to make this attribute type a field of std.builtin.Type.Pointer, which I'd be happy to do with this PR (or open a new one) if it's preferable.

[mlugg]

I considered this while working on the std migration for #23733, but I think we should hold off for now, because I suspect we'll want to change std.builtin.Type instead so that @typeInfo returns data in a more useful form.

[andrewrk]

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