Explorer: Add initial initializing expression support for variable declaration (#2907)

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

explorer/ast/expression.cpp

@@ -31,6 +31,7 @@ auto IntrinsicExpression::FindIntrinsic(std::string_view name,
       {{"print", Intrinsic::Print},
        {"new", Intrinsic::Alloc},
        {"delete", Intrinsic::Dealloc},
+       {"print_allocs", Intrinsic::PrintAllocs},
        {"rand", Intrinsic::Rand},
        {"implicit_as", Intrinsic::ImplicitAs},
        {"implicit_as_convert", Intrinsic::ImplicitAsConvert},
@@ -62,6 +63,8 @@ auto IntrinsicExpression::name() const -> std::string_view {
       return "__intrinsic_new";
     case IntrinsicExpression::Intrinsic::Dealloc:
       return "__intrinsic_delete";
+    case IntrinsicExpression::Intrinsic::PrintAllocs:
+      return "__intrinsic_print_allocs";
     case IntrinsicExpression::Intrinsic::Rand:
       return "__intrinsic_rand";
     case IntrinsicExpression::Intrinsic::ImplicitAs: