Make AttributeSet::getFnAttributes() return an AttributeSet instead of an Attribute.

Make AttributeSet::getFnAttributes() return an AttributeSet instead of an Attribute.

This is more code to isolate the use of the Attribute class to that of just
holding one attribute instead of a collection of attributes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173094 91177308-0d34-0410-b5e6-96231b3b80d8