CodeGen: Cleanup ABIArgInfo

ABIArgInfo was pretty hard to grok, it had data members with names like
"BoolData0" and "UIntData".  Constructors with lots of bool arguments.
Reusing the same data member for multiple purposes.

It was also wasteful with memory, this new layout is 8 bytes smaller.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207894 91177308-0d34-0410-b5e6-96231b3b80d8