Show breakpoint locations of breakpoints which were ignored during :force (#2950)
GHCi is split up into 2 major parts: The user-interface (UI) and the byte-code interpreter. With `-fexternal-interpreter` they even run in different processes. Communication between the UI and the Interpreter (called `iserv`) is done using messages over a pipe. This is called `Remote GHCI` and explained in the Note [Remote GHCi] in `compiler/ghci/GHCi.hs`. To process a `:force` command the UI sends a `Seq` message to the `iserv` process. Then `iserv` does the effective evaluation of the value. When during this process a breakpoint is hit, the `iserv` process has no additional information to enhance the `Ignoring breakpoint` output with the breakpoint location. To be able to print additional breakpoint information, there are 2 possible implementation choices: 1. Store the needed information in the `iserv` process. 2. Print the `Ignoring breakpoint` from the UI process. For option 1 we need to store the breakpoint info redundantely in 2 places and this is bad. Therfore option 2 was implemented in this MR: - The user enters a `force` command - The UI sends a `Seq` message to the `iserv` process. - If processing of the `Seq` message hits a breakpoint, the `iserv` process returns control to the UI process. - The UI looks up the source location of the breakpoint, and prints the enhanced `Ignoring breakpoint` output. - The UI sends a `ResumeSeq` message to the `iserv` process, to continue forcing.