Rename @HotSpotIntrinsicCandidate to @IntrinsicCandidate and move it to the jdk.internal.vm.annotation package

Mark Reinhold suggested during the review of JDK-8137173:

"We don't want to create another sun.misc-like dumping ground
with jdk.internal. We should prefer small, descriptive subpackages
for things like this so that, when they are (inevitably) exposed via
-XaddExports, their exposure can be limited.

I suggest putting this into jdk.internal.vm.annotation, which is also a
good place for the ReservedStackAccess annotation envisioned in JEP 270
(http://openjdk.java.net/jeps/270)."

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Additional suggestion from Mark: Please rename the annotation to
@IntrinsicCandidate There's no need for the "HotSpot" prefix any more
since this annotation will now be in a package that is reserved for
VM-specific annotations.

(Sorry for this late suggestion; this just came up in a parallel
 discussion of the JEP 270's @ReservedStackAccess annotation.)