CodeGenObjCXX: handle inalloca appropriately for msgSend variant

objc_msgSend_stret takes a hidden parameter for the returned structure's
address for the construction.  When the function signature is rewritten
for the inalloca passing, the return type is no longer marked as
indirect but rather inalloca stret.  This enhances the test for the
indirect return to check for that case as well.  This fixes the
incorrect return classification for Windows x86.

llvm-svn: 326362