Remove the 'contains' methods in favor of the 'operator==' method.

Remove the 'contains' methods in favor of the 'operator==' method.

The 'operator==' method is a bit clearer and much less verbose for somethings
that should have only one value. Remove from the AttrBuilder for consistency.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171442 91177308-0d34-0410-b5e6-96231b3b80d8