Remove the Function::getFnAttributes method in favor of using the AttributeSet

Remove the Function::getFnAttributes method in favor of using the AttributeSet
directly.

This is in preparation for removing the use of the 'Attribute' class as a
collection of attributes. That will shift to the AttributeSet class instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171253 91177308-0d34-0410-b5e6-96231b3b80d8