[Draghi] has given global investors what they have clamored for
This is grammatically correct. It is plausible to say that investors have clamoured for it, and Draghi has given it.
However, it does not tell us the order in which the events occurred. Perhaps investors wanted it briefly ten years ago and then lost interest; perhaps they wanted it a little after breakfast today, despite Draghi's announcement.
Because of the context, it's reasonably clear that's what is meant is:
Draghi has given global investors something which investors had been consistently requesting for a long period of time up to the point the announcement.
And it would be somewhat more precise and natural to say:
[Draghi] has given global investors what they had been clamouring for
(though that doesn't absolutely rule out the briefly 10 years ago)
[Draghi] has given global investors what they were clamouring for
Stylistically, I suspect it may have been done for effect:
[Draghi] has given global investors what they have suffered for
In this case 'suffered' might imply either a single great act of suffering or bearing a burden for a long time, and the perfect tense would be a subtle understatement.
However, clamour is already slightly hyperbolic (most investors are not actually shouting loudly at Draghi), so trying at the same time do understatement with it leaves the sentence with an awkward quality to it.