Add partial support for initializing expressions for variable declaration. This is based on https://github.com/carbon-language/carbon-lang/pull/2006, which introduces expression categories, and how it is possible to convert to/from those different categories.
## Functional changes
* Initializing expressions initialize directly the provided storage when used to initialize a variable.
* Allows initializing expressions to avoid a copy when using `[var|let] name: type = call_expression(...)` by initializing `name` in-place.
* Support `returned var: ...` and `return <expr>`
* Support nested initializing expressions
## Main implementation changes
* Updated PatternMatch logic to handle expression categories
* Updated `VariableDefinition` interpreter statement to allocate and pass a location to initializing expressions
* Update statement actions to allow passing an allocation, used by return expr or returned var
* Modified the RuntimeScope API to be one step closer to the memory model we want to have
* Remove `GetAllocationId` and older `Bind` which don't apply
* New set of tests to highlight those different situations
* Added a new intrinsic to print the allocation stack (and make sure we behave correctly, beyond visible side effects)
## Next work
* Dedicated `Action` to retrieve expression category information in the interpreter (https://github.com/carbon-language/carbon-lang/pull/2927)
* Avoid copies when initializing value expression from reference expression and prevent mutations for the duration of the "pinning" (https://github.com/carbon-language/carbon-lang/pull/2927)
* Avoid unnecessary copies from value expression to value expression, after ensuring that even value expression temporaries are registered for destruction.
* Avoid unnecessary copies when binding function arguments
