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/cfe/trunk@171254 91177308-0d34-0410-b5e6-96231b3b80d8