These changes integrate let declarations into the template pipeline. This involves a few operations:
* Producing a `declareLet` instruction call at creation time to initialize the declaration.
* Producing a `storeLet` instruction call in the place of the let declaration, including the necessary `advance` calls beforehand.
* For let declarations used within their declaration view, moving the `const` to be placed right after the `storeLet` call to ensure the their value has been computed.
* For let declarations that are _only_ used in their declaration view, removing the `storeLet` call and inlining the expression into the constant statement.
PR Close#56299
64990a50ed