Summary: Adds functionality to assemble an indexed source map to the new Buck integration. This implementation supports startup section optimisations. Hooking it up, and grouping optimisations will be in follow-ups.
Reviewed By: jeanlauliac
Differential Revision: D5106985
fbshipit-source-id: cc4c6ac8cfe4e718fc8bb2a8a93cb88914c92e0b