Add NeXT runtime support for generating methods.

Change CodeGenFunction::EmitParmDecl to take either a ParmVarDecl or an
  ImplicitParamDecl.

Drop hasAggregateLLVMType from CodeGenModule.cpp (use version in
  CodeGenFunction).

Change the Objective-C method generation to use EmitParmDecl for
  implicit parameters.

llvm-svn: 54838