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.