[IR] Take func, ret, and arg attrs separately in AttributeList::get

[IR] Take func, ret, and arg attrs separately in AttributeList::get

This seems like a much more natural API, based on Derek Schuff's
comments on r300015. It further hides the implementation detail of
AttributeList that function attributes come last and appear at index
~0U, which is easy for the user to screw up. git diff says it saves code
as well: 97 insertions(+), 137 deletions(-)

This also makes it easier to change the implementation, which I want to
do next.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300153 91177308-0d34-0410-b5e6-96231b3b80d8