Insert checks for enum discriminants when debug assertions are enabled

Similar to the existing nullpointer 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@.

Author: 1c3t3a

src/lib.rs

@@ -165,7 +165,7 @@ 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",