Improve x86 ABI compatibility.

 - Enables use of ABIArgInfo::Expand when needed. This greatly
   improves our x86 ABI compatibility.

 - As the infrastructure for target specific ABI handling isn't built
   yet, this change means ABI compatibility on other platforms is once
   again broken in a different way than before.

 - Upcoming: Figure out how to refactor ABI handling into
   targets. More documentation.

llvm-svn: 56293