This crate was failing to compile due to dead_code/unused_imports
warnings. This commits disables these two lints for these targets.

rustc_trans: don't lose the cross-crate DefId, MIR trans needs it.

We might have been missing out on some issues because MIR trans was never being used cross-crate.

cc @rust-lang/compiler

…komatsakis

Automatically enable -Zorbit if -Zincremental is specified.

Fixes #34973

r? @nikomatsakis

core: fix `cargo build` for targets with "max-atomic-width": 0

This crate was failing to compile due to dead_code/unused_imports
warnings. This commits disables these two lints for these targets.

---

r? @alexcrichton
cc @Amanieu is `cfg(target_has_atomic = "8")` the right `cfg` to use? I think that all targets that support some form of atomics will at a minimum support byte level atomics.

FWIW, the only thing that's left in `sync::atomic` for these targets is `Ordering` and the `fence` function.

Add -mrelax-relocations=no hacks to fix musl build

* this is just a start, dunno if it will work, but I'll just push it out to get feedback (my rust is still building 😢)
* I don't know much about rustbuild, so i just added that flag in there. it's a total hack, don't judge me
* I suspect the places in the musl .mk files are sufficient (but we may also need it present when building std), I'm not sure, needs more testing.

the code has no tests and will just bitrot by itself.

this is a [breaking-change]

Audit C++ types in rustllvm

cc @eddyb

Fixes #35131

Support removed LLVM intrinsics by invoking its AutoUpgrade mechanism.

Turns out that LLVM sometimes renames platform intrinsics or replaces them with first-class instructions.
For example, signed minimum became `select (icmp SLT, a, b), a, b` where `a` and `b` are vectors.

This is blocking the Servo rustup ([relevant failure](http://build.servo.org/builders/windows-dev/builds/226/steps/compile/logs/stdio)), as they're using a few such intrinsics.
The fix in this PR is to invoke LLVM's own `AutoUpgrade` mechanism to do the replacements.