Maintain cost-centre stacks in the interpreter

Summary:
Breakpoints become SCCs, so we have detailed call-stack info for
interpreted code.  Currently this only works when GHC is compiled with
-prof, but D1562 (Remote GHCi) removes this constraint so that in the
future call stacks will be available without building your own GHCi.

How can you get a stack trace?

* programmatically: GHC.Stack.currentCallStack
* I've added an experimental :where command that shows the stack when
  stopped at a breakpoint
* `error` attaches a call stack automatically, although since calls to
  `error` are often lifted out to the top level, this is less useful
  than it might be (ImplicitParams still works though).
* Later we might attach call stacks to all exceptions

Other related changes in this diff:

* I reduced the number of places that get ticks attached for
  breakpoints.  In particular there was a breakpoint around the whole
  declaration, which was often redundant because it bound no variables.
  This reduces clutter in the stack traces and speeds up compilation.

* I tidied up some RealSrcSpan stuff in InteractiveUI, and made a few
  other small cleanups

Test Plan: validate

Reviewers: ezyang, bgamari, austin, hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1595

GHC Trac Issues: #11047

1 files changed, 3 insertions, 2 deletions

diff --git a/rts/Disassembler.c b/rts/Disassembler.c
index 7e3529bb7f..2e1790ef8d 100644
--- a/rts/Disassembler.c
+++ b/rts/Disassembler.c
@@ -67,8 +67,9 @@ disInstr ( StgBCO *bco, int pc )
    switch (instr & 0xff) {
       case bci_BRK_FUN:
          debugBelch ("BRK_FUN  " );  printPtr( ptrs[instrs[pc]] );
-         debugBelch (" %d ", instrs[pc+1]); printPtr( ptrs[instrs[pc+2]] ); debugBelch("\n" );
-         pc += 3;
+         debugBelch (" %d ", instrs[pc+1]); printPtr( ptrs[instrs[pc+2]] );
+         debugBelch(" %s\n", ((CostCentre*)(literals[instrs[pc+3]]))->label);
+         pc += 4;
          break;
       case bci_SWIZZLE:
          debugBelch("SWIZZLE stkoff %d by %d\n",