Track reason for creating a ReifyShim

KCFI needs to be able to tell which kind of `ReifyShim` it is examining
in order to decide whether to use a concrete type (`FnPtr` case) or an
abstract case (`Vtable` case). You can *almost* tell this from context,
but there is one case where you can't - if a trait has a method which is
*not* `#[track_caller]`, with an impl that *is* `#[track_caller]`, both
the vtable and a function pointer created from that method will be
`ReifyShim(def_id)`.

Currently, the reason is optional to ensure no additional unique
`ReifyShim`s are added without KCFI on. However, the case in which an
extra `ReifyShim` is created is sufficiently rare that this may be worth
revisiting to reduce complexity.

compiler/rustc_middle/src/ty/mod.rs

@@ -88,7 +88,7 @@ pub use self::context::{
     tls, CtxtInterners, CurrentGcx, DeducedParamAttrs, Feed, FreeRegionInfo, GlobalCtxt, Lift,
     TyCtxt, TyCtxtFeed,
 };
-pub use self::instance::{Instance, InstanceDef, ShortInstance, UnusedGenericParams};
+pub use self::instance::{Instance, InstanceDef, ReifyReason, ShortInstance, UnusedGenericParams};
 pub use self::list::List;
 pub use self::parameterized::ParameterizedOverTcx;
 pub use self::predicate::{