There was a bug in the logic for emitting initial IR, such that it was neglecting to emit "methods" (member functions) unless they were also referenced by a non-member (global) function, or were needed to satisfy an interface requirement. This would only matter for `import`ed modules, since for non-`import`ed code, anything relevant would be referenced by the entry point so that the problem would never surface.

This change fixes the underlying problem by adding a step to the IR lowering pass called `ensureAllDeclsRec` that makes sure that not only global-scope declarations, but also anything nested under a `struct` type gets emitted to the initial IR module.

There are also a few unrelated fixes in this PR, which are things I ran into while making the fix:

* Deleted support for the (long gone) `IRDeclRef` type in our `slang.natvis` file

* Added support for visualizing the value of IR string and integer literals when they appear in the debugger

* Fixed IR dumping logic to not skip emitting `struct` and `interface` instructions. Switching those to inherit from `IRType` accidentally affected how they get printed in IR dumps by default.

* Fixed up the IR linking logic so that it correctly takes `[export]` decorations into account, so that an exported definition will always be taken over any other (unless the latter is more specialized for the target). I initially implemented this in an attempt to fix the original issue, but found it wasn't a fix for the root cause. It is still a better approach than what was implemented previously, so I'm leaving it in place.