C++ Interop: Toolchain Implementation for Function Calls

[bricknerb]

This proposal details the toolchain implementation for calling imported C++
functions from Carbon. It covers how C++ overload sets are handled, the process
of overload resolution leveraging Clang, and the generation of "thunks"
(intermediate functions) when necessary to bridge Application Binary Interface
(ABI) differences between Carbon and C++.

[bricknerb]

FWIW, I've started documenting some of this in #6358.

[zygoloid]

There seems to be some overlap between this and https://docs.google.com/document/d/1KUxumZtNe3mY3TsjW2s_ZADOlAaFlrtsLKHVILtqIaM/edit -- but actually not very much. (And Chandler or I should turn that document into a proposal...)

Also, this document reads to me like it's mostly describing a design for implementing C++ function calls in the toolchain in particular (thunks, ABI, type representation, etc. are concerns at that level, I think), so perhaps we could very slightly adjust the framing here to be about that. I think that would also remove the appearance of overlap between the two proposals.

[bricknerb]

There seems to be some overlap between this and https://docs.google.com/document/d/1KUxumZtNe3mY3TsjW2s_ZADOlAaFlrtsLKHVILtqIaM/edit -- but actually not very much. (And Chandler or I should turn that document into a proposal...)

Also, this document reads to me like it's mostly describing a design for implementing C++ function calls in the toolchain in particular (thunks, ABI, type representation, etc. are concerns at that level, I think), so perhaps we could very slightly adjust the framing here to be about that. I think that would also remove the appearance of overlap between the two proposals.

Done.