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, 1 insertions, 2 deletions

diff --git a/testsuite/tests/ghci.debugger/scripts/break018.script b/testsuite/tests/ghci.debugger/scripts/break018.script
index 0a4c70ef5a..a30af6bfd1 100644
--- a/testsuite/tests/ghci.debugger/scripts/break018.script
+++ b/testsuite/tests/ghci.debugger/scripts/break018.script
@@ -1,9 +1,8 @@
 -- Check mdo statements: availability of local bindings.
--- Maybe we should not want to put in scope the things binded in the mdo scope, to avoid silliness. 
+-- Maybe we should not want to put in scope the things binded in the mdo scope, to avoid silliness.
 
 :set -XRecursiveDo
 :l ../mdo.hs
 :st l2dll "hello world"
 :st
 :st
-:st