Represent pass_object_size attrs in ExtParameterInfo

The goal of this is to fix a bug in modules where we'd merge
FunctionDecls that differed in their pass_object_size attributes. Since
we can overload on the presence of pass_object_size attributes, this
behavior is incorrect.

We don't represent `N` in `pass_object_size(N)` as part of
ExtParameterInfo, since it's an error to overload solely on the value of
N. This means that we have a bug if we have two modules that declare
functions that differ only in their pass_object_size attrs, like so:

// In module A, from a.h
void foo(char *__attribute__((pass_object_size(0))));

// In module B, from b.h
void foo(char *__attribute__((pass_object_size(1))));

// In module C, in main.c
#include "a.h"
#include "b.h"

At the moment, we'll merge the foo decls, when we should instead emit a
diagnostic about an invalid overload. We seem to have similar (silent)
behavior if we overload only on the return type of `foo` instead; I'll
try to find a good place to put a FIXME (or I'll just file a bug) soon.

This patch also fixes a bug where we'd not output the proper extended
parameter info for declarations with pass_object_size attrs.

llvm-svn: 296076