[m-dev.] for review: fix stack traces

[Zoltan Somogyi]

> compiler/continuation_info.m:
> compiler/mercury_compile.m:
> 	Collect continuation label information for stack traces, but
> 	without the liveness information that is needed for agc.
> 
> compiler/stack_layout.m:
> 	Add a comment stating that changes in this file may need to be
> 	reflected in runtime/mercury_stack_layout.h

These changes are fine.

Zoltan.