Rename AttributeSetImpl::NumAttrs and AttributeSetImpl::getNumAttributes to...

Rename AttributeSetImpl::NumAttrs and AttributeSetImpl::getNumAttributes to reflect that they work on slots rather than attributes. NFC

Summary: The current naming not only doesn't convey the meaning of what this does, but worse, it convey the wrong meaning. This was a major source of confusion understanding the code, so I'm applying the boy scout rule here and making it better after I leave.

Reviewers: void, bkramer, whitequark

Differential Revision: http://reviews.llvm.org/D21264

llvm-svn: 272725