Apply noundef attribute to all scalar types which do not permit raw init

Beyond `&`/`&mut`/`Box`, this covers `char`, discriminants, `NonZero*`, etc.
All such types currently cause a Miri error if left uninitialized,
and an `invalid_value` lint in cases like `mem::uninitialized::<char>()`

Note that this _does not_ change whether or not it is UB for `u64` (or
other integer types with no invalid values) to be undef.

compiler/rustc_middle/src/ty/layout.rs

@@ -3053,6 +3053,11 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                 return;
             }
 
+            // Scalars which have invalid values cannot be undef.
+            if !scalar.is_always_valid(self) {
+                attrs.set(ArgAttribute::NoUndef);
+            }
+
             // Only pointer types handled below.
             if scalar.value != Pointer {
                 return;