Refactor call & function handling in trans, enable MIR bootstrap.
Non-Rust and Rust ABIs were combined into a common codepath, which means:
* The ugly `__rust_abi` "clown shoes" shim for C->Rust FFI is gone, fixes#10116.
* Methods, *including virtual ones* support non-Rust ABIs, closes#30235.
* Non-Rust ABIs also pass fat pointers in two arguments; the result should be identical.
* Zero-sized types are never passed as arguments; again, behavior shouldn't change.
Additionally, MIR support for calling intrinsics (through old trans) was implemented.
Alongside assorted fixes, it enabled MIR to launch 🚀 and do a *complete* bootstrap.
To try it yourself, `./configure --enable-orbit` *or* `make RUSTFLAGS="-Z orbit"`.
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