[clang] Add an API to retrieve implicit constructor arguments.

Summary:
This is needed in Swift for C++ interop -- see here for the corresponding Swift change:

https://github.com/apple/swift/pull/30630

As part of this change, I've had to make some changes to the interface of CGCXXABI to return the additional parameters separately rather than adding them directly to a `CallArgList`.

Reviewers: rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79942