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.