Put Local, Static and Promoted as one Base variant of Place
Related to #52708
The `Place` 2.0 representation use a `Base` variant for `Local`, `Static` and `Promoted` so we start making this change in the current `Place` to make the following steps simpler.
r? @oli-obk
Add unstable option to ignore should_panic tests
Add an unstable option `--exclude-should-panic` to libtest to workaround https://github.com/rust-lang/miri/issues/636
?r @oli-obk
cc @RalfJung
rustc: Update LLVM, remove dead wasm code
This commit updates the LLVM branch to the rebased version of the
upstream release/8.x branch. This includes a wasm patch which means that
the `rewrite_imports` pass in rustc is no longer needed (yay!) and we
can instead rely on `wasm-import-module`, an attribute we're already
emitting, to take care of all the work.
Support defining C compatible variadic functions
## Summary
Add support for defining C compatible variadic functions in unsafe rust with
`extern "C"` according to [RFC 2137].
## Details
### Parsing
When parsing a user defined function that is `unsafe` and `extern "C"` allow
variadic signatures and inject a "spoofed" `VaList` in the new functions
signature. This allows the user to interact with the variadic arguments via a
`VaList` instead of manually using `va_start` and `va_end` (See [RFC 2137] for
details).
### Codegen
When running codegen for a variadic function, remove the "spoofed" `VaList`
from the function signature and inject `va_start` when the arg local
references are created for the function and `va_end` on return.
## TODO
- [x] Get feedback on injecting `va_start/va_end` in MIR vs codegen
- [x] Properly inject `va_end` - It seems like it should be possible to inject
`va_end` on the `TerminatorKind::Return`. I just need to figure out how
to get the `LocalRef` here.
- [x] Properly call Rust defined C variadic functions in Rust - The spoofed
`VaList` causes problems here.
Related to: #44930
r? @ghost
[RFC 2137]: https://github.com/rust-lang/rfcs/blob/master/text/2137-variadic.md
Make `Unique::as_ptr` const without feature attribute as it's unstable
Make `NonNull::dangling` and `NonNull::cast` const with `feature = "const_ptr_nonnull"`
