Remove Attribute::hasAttributes() and make Attribute::hasAttribute() private.

The Attribute::hasAttributes() is kind of meaningless since an Attribute can
have only one attribute. And we would rather people use the 'operator=='
instead of Attribute::hasAttribute().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174026 91177308-0d34-0410-b5e6-96231b3b80d8