swift234

Implement clang::ABIArgInfo::Expand correctly when matching
the C calling convention.

You might notice the absence of tests.  Older versions of Clang
used Expand for ARM64 homogeneous aggregates.  Trunk Clang no
longer does this, and indeed, on Apple platforms we no longer
use Expand to pass any kind of argument at all.  The only target
in trunk that uses it is the Windows target's vectorcall
alternative CC, which I have no way to convince Swift to
generate a call against.  Therefore this code is completely
dead in trunk, and thus untestable.

Removing Expand completely from Clang would be the ideal solution,
but that's politically tricky because this is a part of Clang
that's frequently edited by people supporting out-of-tree backends.
I've started the process, but until then, if we're going to have
dead code, it ought to at least be correct dead code.

This commit fixes rdar://19199427, or at least it will
when it's merged to branches that actually use Expand.

Swift SVN r24949