Fix an issue with explicit enum tag types (#1495)

The basic problem here was that in a declaration like:

```hlsl
enum Color : uint { Red, Orange, ... }
```

The `: uint` bit is represented as an `InheritanceDecl`, because that is what we use to represent the syntactic form of inheritance clauses like that. At the point where we parse the `InheritanceDecl` we don't yet know whether it represents a base interface or a "tag type" like `uint` in this case.

The root problem that is then created is: an `enum` type is *not* a subtype of its "tag type," and treating it like a subtype can create problems.

The main problem that arises is that looking in a type like `Color` will find both the members of color *and* the members of `uint`. In the case of things like `__init` declarations, that creates a problem where the `Color` type has two different `__init`s that take a `uint`:

* The one it inherits from `uint` via that `InheritanceDecl` (even though it shouldn't)
* The one it gets via an extension just for conforming to `__EnumType` (a non-user-exposed `interface` in the standard library)

Because both of those `__init`s are inherited, neither is preferred over the other one and they create an ambiguity if somebody tries to write:

```hlsl
uint u = ...;
Colorc = Color(u);
```

The solution used in this PR is to add a compiler-internal modifier to the `InheritanceDecl` that introduces a "tag type" to an `enum`, in an early phase of checking (one of the ones that occurs before it is legal to enumerate the bases of a type). Then the lookup process is modified to ignore `InheritanceDecl`s with that modifier when doing lookup in super-types (since the declaration does *not* indicate a subtype/supertype relationship).

This appears to get the basic feature working again, although it is possible that there are other parts of the compiler that use `InheritanceDecl`s and mistakently assume that all `InheritanceDecl`s introduce subtype/supertype relationships. We probably need to do a significant audit of the code to start being more clear about the nature of the relationships such declarations introduce. Such steps are left to future changes.

Co-authored-by: Yong He <yonghe@outlook.com>

0 files changed, 0 insertions, 0 deletions