`Value` had a number of methods that were never used (like `set()`), so just remove them.
Remove `Type`, this was also unused, and was unncessarily complex to maintain.

Add `Variables` interface that allows other composite value implementations that are not `VariableContainer`.

Break the inheritance of `VariableContainer` from `Value`, as this forces pointer casting and doesn't work with the `Variables` interface.

Add `Struct` which is an implementation of `Value` that implements the new `children()` method to return the provided `Variables`.

Sets the groundwork for the overhauled debugger implementation.

Note: The changes to `SpirvShaderDebugger.cpp` are a least-effort set of changes to make things compile.
A significant amount of this file (along with these changes) will be reimplemented in a followup change.

Bug: b/145351270
Change-Id: Ic3a641a246737b1f82c786fa8c1ec75700b2a71a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48693
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>