add explicit flag to replace_reason calls (part 1)
add explicit flag to replace_reason calls (part 1)

Summary: `def_loc` would be dropped by default when replacing descriptions. I want to make it such that either `def_loc` and `annot_loc` are preserved (indicating some sort of derived reason), or they are explicitly dropped (indicating a new reason). APIs will be renamed later.

Reviewed By: panagosg7

Differential Revision: D17054268

fbshipit-source-id: 90b8fdeb77ab4f2753ddddf665e4f13a666ac8c0

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow