passing along source location when building nodes

The problem I had before was due to transformations not keeping the source locations in the final nodes. There are a few key places that need to copy along the source locs.

What's the right pattern to do this with builders? Do I need to assign the node to a variable and then do something like `newNode.loc = oldNode.loc`? Or can I pass along the source locations to the builders somehow?

Trace the transformations and builders that create the final nodes, then inspect how source locations are currently represented and preserved. Determine the intended builder pattern for carrying source locations through transformations; the work is done when the approach is defined and the affected transformations consistently retain their source locations.

Written by the indexing model from the issue text.