Convert typeIncompatible to return an AttributeSet.

There are still places which treat the Attribute object as a collection of
attributes. I'm systematically removing them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173990 91177308-0d34-0410-b5e6-96231b3b80d8
Remove addRetAttributes and addFnAttributes, which aren't useful abstractions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173992 91177308-0d34-0410-b5e6-96231b3b80d8
PPC QPX requires a 32-byte aligned stack

On systems which support the QPX vector instructions, the stack must be
32-byte aligned.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173993 91177308-0d34-0410-b5e6-96231b3b80d8
Added a unit test for r173983 that verifies that Target.isiOS() works correctly.

As a bonus I put in some extra checks to make sure that we are identifying the
machine word of various Mac OS X/iOS targets appropriately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173994 91177308-0d34-0410-b5e6-96231b3b80d8