* Support conversion from int/uint to enum types

The basic feature here is tiny, and is summarized in the code added to the stdlib:

```
extension __EnumType
{
	__init(int val);
	__init(uint val);
}
```

The front-end already makes all `enum` types implicitly conform to `__EnumType` behind the scenes, and this `extension` makes it so that all such types inherit some initializers (`__init` declarations, aka. "constructors") that take `int` and `uint`.

(Note: right now all `__init` declarations in Slang are assumed to be implemented as intrinsics using `kIROp_Construct`. This obviously needs to change some day, especially so that we can support user-defined initializers.)

Actually making this *work* required a bit of fleshing out pieces of the compiler that had previously been a bit ad hoc to be a bit more "correct." Most of the rest of this description is focused on those details, since the main feature is not itself very exciting.

When overload resolution sees an attempt to "call" a type (e.g., `MyType(3.0)`) it needs to add appropriate overload candidates for the initializers in that type, which may take different numbers and types of parameters. The existing code for handling this case was using an ad hoc approach to try to enumerate the initializer declarations to consider, which might be found via inheritance, `extension` declarations, etc.

In practice, the ad hoc logic for looking up initializers was just doing a subset of the work that already goes into doing member lookup. Changing the code so that it effectively does lookup for `MyType.__init` allows us to look up initializers in a way that is consistent with any other case of member lookup. Generalizing this lookup step brings us one step closer to being able to go from an `enum` type `E` to an initializer defined on an `extension` of an `interface` that `E` conforms to.

One casualty of using the ordinary lookup logic for initializers is that we used to pass the type being constructed down into the logic that enumerated the initializers, which made it easier to short-circuit the part of overload resolution that usually asks "what type does this candidate return."

It might seem "obvious" that an initializer/constructor on type `Foo` should return a value of type `Foo`, but that isn't necessarily true.
Consider the `__BuiltinFloatingPointType` interface, which requires all the built-in floating-point types (`float`, `double`, `half`) to have an initializer that can take a `float`.
If we call that interface in a generic context for `T : __BuiltinFloatingPointType`, then we want to treat that initializer as returning `T` and not `__BuiltinFloatingPointType`.
Without the ad hoc logic in initializer overload resolution, this is the exact problem that surfaced for the stdlib definition of `clamp`.

The solution to the "what type does an initializer return" problem was to introduce a notion of a `ThisType`, which refers to the type of `this` in the body of an interface.
More generally, we will eventually want to have the keyword `This` be the type-level equivalent of `this`, and be usable inside any type.
The `calcThisType` function introduced here computes a reasonable `Type` to represent the value of `This` within a given declaration.
Inside of concrete type it refers to the type itself, while in an `interface` it will always be a `ThisType`.
The existing `ThisTypeSubstitution`s, previously only applied to associated types, now apply to `ThisType`s as well, in the same situations.

The next roadblock for making the simple declarations for `__EnumType` work was that the lookup logic was only doing lookup through inheritance relationships when the type being looked up in was an `interface`.
The logic in play was reasonable: if you are doing lookup in a type `T` that inherits from `IFoo`, then why bother looking for `IFoo::bar` when there must be a `T::bar` if `T` actually implements the interface?

The catch in this case is that `IFoo::bar` might not be a requirement of `IFoo`, but rather a concrete method added via an `extension`, in which case `T` need not have its own concrete `bar`.
The simple/obvious fix here was to make the lookup logic always include inherited members, even when looking up through a concrete type.

Of course, if we allow lookup to see `IFoo::bar` when looking up on `T`, then we have the problem that both `T::bar` and `IFoo::bar` show up in the lookup results, and potentially lead to an "ambiguous overload" error.
This problem arises for any interface rquirement (so both methods and associated types right now).
In order to get around it, I added a somewhat grungy check for comparing overload candidates (during overload resolution) or `LookupResultItem`s (during resolution of simple overloaded identifiers) that considers a member of a concrete type as automatically "better" than a member of an interface.
The Right Way to solve this problem in the long run requires some more subtlety, but for now this check should Just Work.

One final wrinkle is that due to our IR lowering pass being a bit overzealous, we currently end up trying to emit IR for those new `__init` declarations, which ends up causing us to try and emit IR for a `ThisType`.
That is a case that will require some subtlty to handle correctly down the line, for for now we do the expedient thing and emit the `ThisType` for `IFoo` as `IFoo` itself, which is not especially correct, but doesn't matter since the concrete initializer won't ever be called.

* testing: add more debug output to Unix process launch function

* testing: increase timeout when running command-line tests