Make setABIAPCS and setABIAAPCS easier to reason about.

These functions now always set the same variables in the same order and they
don't overlap with thep constructor.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197604 91177308-0d34-0410-b5e6-96231b3b80d8