Refactor handling of calls:

 - Added CodeGenFunction::EmitCall which just takes the callee, return
   type, and a list of (Value*,QualType) pairs.
 - Added CodeGenFunction::EmitCallArg which handles emitting code for
   a call argument and turning it into an appropriate
   (Value*,QualType) pair.
 - Changed Objective-C runtime interface so that the actual emission
   of arguments for message sends is (once again) done in the code to
   emit a message send.

No intended functionality change, this is prep work for better ABI
support and for Objective-C property setter support.

llvm-svn: 55560