[MS ABI] Improve our mangling of pass_object_size

We didn't add the artificial pass_object_size arguments to the
backreference map which bloated the size of manglings which involved
pass_object_size with duplicate types.

This lets us go from:
?qux@PassObjectSize@@YAHQAHW4__pass_object_size1@__clang@@0W4__pass_object_size1@3@@Z

to:
?qux@PassObjectSize@@YAHQAHW4__pass_object_size1@__clang@@01@Z

llvm-svn: 256622