Use ABIArgInfo for decisions about function arguments (not just return

value).
 - Added ABIArgInfo::ByVal (mostly supported) and ABIArgInfo::Expand
   (asserted out).

 - Added classifyArgumentType which currently just uses
   ABIArgInfo::Default or ByVal. This nearly matches old behavior, but
   we now set ByVal in a few situations we may have left it off before
   (on complex, for example).

llvm-svn: 56086