[rfc][gold plugin] Fix pr19901

[Duncan P. N. Exon Smith]

> On 2014-Aug-11, at 17:09, Nick Lewycky <nlewycky at google.com> wrote:
> 
> +static void keepGlobalValue(GlobalValue &GV) {
> 
> I've been pontificating over what it means to have llvm.used name a function with a discardable linkage. The idea is that it would be one way to communicate to internalize that it should form weak linkage out of it. I'm not sure whether that's safe, it means that attribute used would run the risk of changing linkages.

I think -internalize should leave alone anything in `@llvm.used` -- are
you suggesting otherwise, or have I misunderstood?