Auto merge of #141759 - 1c3t3a:discriminants-query, r=saethlin

Insert checks for enum discriminants when debug assertions are enabled

Similar to the existing null-pointer and alignment checks, this checks for valid enum discriminants on creation of enums through unsafe transmutes. Essentially this sanitizes patterns like the following:
```rust
let val: MyEnum = unsafe { std::mem::transmute<u32, MyEnum>(42) };
```

An extension of this check will be done in a follow-up that explicitly sanitizes for extern enum values that come into Rust from e.g. C/C++.

This check is similar to Miri's capabilities of checking for valid construction of enum values.

This PR is inspired by saethlin@'s PR
rust-lang/rust#104862. Thank you so much for keeping this code up and the detailed comments!

I also pair-programmed large parts of this together with vabr-g@.

r? `@saethlin`

Author: bors

src/lib.rs

@@ -168,7 +168,7 @@ pub const MIRI_DEFAULT_ARGS: &[&str] = &[
     "-Zmir-emit-retag",
     "-Zmir-preserve-ub",
     "-Zmir-opt-level=0",
-    "-Zmir-enable-passes=-CheckAlignment,-CheckNull",
+    "-Zmir-enable-passes=-CheckAlignment,-CheckNull,-CheckEnums",
     // Deduplicating diagnostics means we miss events when tracking what happens during an
     // execution. Let's not do that.
     "-Zdeduplicate-diagnostics=no",