Use UDAs for GDC attributes

[jerro]

I think it would be a good idea to replace GDC's pragma attribute 
with UDAs. I I have an implementation of this at 
https://github.com/jerro/GDC/tree/uda-gcc-attributes. The API 
consists of a module named gcc.attribute that contains a function 
template attribute():

auto attribute(A...)(A args) if(A.length > 0 && is(A[0] == 
string))

This function is used like this:

import gcc.attribute;

@attribute("gdc_attribute_name", arg1, arg2) foo()
{
     ...
}

This adds an attribute gdc_attribute_name(arg, arg2) to foo and 
is equivalent to

pragma(attribute, gdc_attribute_name(arg1, arg2)) foo()
{
     ....
}

The advantage of using UDAs for GDC attributes is that you can 
alias them or use typetuples of attributes, possibly depending on 
the values of template parameters. One possible use case is this:

https://github.com/TurkeyMan/phobos/pull/3#issuecomment-11694942

Using UDAs for attributes would also enable us to remove a bunch 
of code that currently deals with GDC attributes from GDC's 
version of the frontend.

I'd like to hear your opinions and suggestions on this.