Auto merge of #44111 - zackmdavis:feature_attr_error_span, r=nikomatsakis

feature error span on attribute for fn_must_use, SIMD/align reprs, macro reëxport

There were several feature-gated attributes for which the feature-not-available
error spans would point to the item annotated with the gated attribute, when it
would make more sense for the span to point to the attribute itself: if the
attribute is removed, the function/struct/_&c._ likely still makes sense and the
program will compile. (Note that we decline to make the analogous change for
the `main`, `start`, and `plugin_registrar` features, for in those cases it
makes sense for the span to implicate the entire function, of which there is
little hope of using without the gated attribute.)

![feature_attr_error_span](https://user-images.githubusercontent.com/1076988/29746531-fd700bfe-8a91-11e7-9c5b-6f5324083887.png)

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