Fix handling of anonymous aggregate parameters for powerpc*-apple-darwin8.

This fixes bug 15821 similarly to the powerpc64-linux fix for bug 14779.

Patch by David Fang.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181449 91177308-0d34-0410-b5e6-96231b3b80d8