Auto merge of #65947 - eddyb:fn-abi, r=oli-obk,nagisa

rustc: split FnAbi's into definitions/direct calls ("of_instance") and indirect calls ("of_fn_ptr").

After this PR:
* `InstanceDef::Virtual` is only used for "direct" virtual calls, and shims around those calls use `InstanceDef::ReifyShim` (i.e. for `<dyn Trait as Trait>::f as fn(_)`)
  * this could easily be done for intrinsics as well, to allow their reification, but I didn't do it
* `FnAbi::of_instance` is **always** used for declaring/defining an `fn`, and for direct calls to an `fn`
  * this is great for e.g. https://github.com/rust-lang/rust/pull/65881 (`#[track_caller]`), which can introduce the "caller location" argument into "codegen signatures" by only changing `FnAbi::of_instance`, after this PR
* `FnAbi::of_fn_ptr` is used primarily for indirect calls, i.e. to `fn` pointers
  * *not* virtual calls (which use `FnAbi::of_instance` with `InstanceDef::Virtual`)
  * there's also a couple uses where the `rustc_codegen_llvm` needs to declare (i.e. FFI-import) an LLVM function that has no Rust declaration available at all
    * at least one of them could probably be a "weak lang item" instead

As there are many steps, this PR is best reviewed commit by commit - some of which arguably should be in their own PRs, I may have gotten carried away a bit.

cc @nagisa @rkruppe @oli-obk @anp

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