Add a mechanism for collecting information about how many times each event

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Add a mechanism for collecting information about how many times each event
is executed. Later changes will exploit this information, for coverage testing
and for better search strategies in the declarative debugger.

runtime/mercury_stack_layout.h:
	Extend the module layout structure to contain a pointer to an array
	of counters, with one counter for each label layout structure
	corresponding an event.

	Extend label layout structures to contain the label's index into
	this array. The new field fits into a hole in the existing structure
	required by alignment considerations, so the size of label layout
	structures is unaffected.

	The memory cost of the new facility is thus only one word per event.

	Extend the macros for defining label layout structures to take an extra
	argument for the new field.

runtime/mercury_grade.m:
	Update the binary compatibility version number for debug grades,
	since runtimes before this change and the compiler-generated C files
	in debug grades from after this change are not compatible, and neither
	are runtimes after this change and the compiler-generated C files
	in debug grades from before this change.

runtime/mercury_trace_base.[ch]:
	Add a third alternative function to call from MR_trace; besides
	MR_trace_real (in the trace directory) and MR_trace_fake (in this
	module), add MR_trace_count, which returns after incrementing the
	counter of the event's label layout structure.

	Instead of a single variable, MR_trace_enabled, controlling whether
	MR_trace is enabled or not and debugging is enabled or not, use two
	separate variables: MR_trace_func_enabled controlling whether MR_trace
	is enabled or not, and MR_debug_enabled controlling whether debugging
	is enabled or not. MR_trace_func_enabled is always set to the
	disjunction of MR_debug_enabled and a new variable
	MR_trace_count_enabled, the variable controlling whether the facility
	for counting label executions is enabled.

	Add a function for writing out the gathered execution counts.

runtime/mercury_trace_base.[ch]:
trace/mercury_trace_tables.[ch]:
	Move the table of module_infos from mercury_trace_tables to
	mercury_trace_base, since the code to write out execution counts
	needs it.

runtime/mercury_wrapper.c:
	When MERCURY_OPTIONS contains --trace-count and debugging is not,
	enable the mechanism for counting the execution of events, and
	write out the results.

library/exception.m:
library/io.m:
library/table_builtin.m:
runtime/mercury_tabling_preds.h:
trace/mercury_trace_declarative.c:
trace/mercury_trace_external.c:
trace/mercury_trace_vars.c:
	Conform to the changes involving the replacement of MR_trace_enabled.

tools/bootcheck:
	Add an new option, --trace-count, for enabling the new mechanism.

compiler/layout.m:
	Change the compiler's data structures for label layouts and module
	layouts to conform to the changes in mercury_stack_layout.h.

compiler/layout_out.m:
	Update the code for writing out label layouts and module
	layouts.

compiler/stack_layout.m:
	Allocate slots in the execution count array to the label layout
	structures of all events.

	Delete the unnecessary last arguments of `get' field access predicates.

compiler/llds_out.m:
compiler/opt_debug.m:
	Conform to the changes in layout.m.

compiler/opt_debug.m

@@ -531,6 +531,11 @@ dump_layout_name(module_layout_proc_vector(ModuleName), Str) :-
 	ModuleNameStr = sym_name_mangle(ModuleName),
 	string__append_list(["module_layout_proc_vector(", ModuleNameStr, ")"],
 		Str).
+dump_layout_name(module_layout_label_exec_count(ModuleName, NumLabels), Str) :-
+	ModuleNameStr = sym_name_mangle(ModuleName),
+	NumLabelsStr = int_to_string(NumLabels),
+	string__append_list(["module_layout_label_exec_count(",
+		ModuleNameStr, ", ", NumLabelsStr, ")"], Str).
 dump_layout_name(module_layout(ModuleName), Str) :-
 	ModuleNameStr = sym_name_mangle(ModuleName),
 	string__append_list(["module_layout(", ModuleNameStr, ")"], Str).