Construct partially instantiated direct arg functor values.

Construction unifications of partially instantiated values involving direct
argument functors (where the single argument is free) did not generate any code
in both low-level and high-level backends.  Incorrect behaviour could result if
the program tried to deconstruct the value at run-time.

Also, in the LLDS backend, such a construction unification did not enter the
variable into the var_state_map, leading to a compiler abort when the variable
is looked up.

compiler/ml_unify_gen.m:
	Generate code for constructions of a direct arg functor with free
	argument.  This amounts to assigning a variable to a tagged null
	pointer.

compiler/llds.m:
	Add an rval option `mkword_hole', which is like `mkword' but the
	pointer to be tagged is unspecified.

compiler/unify_gen.m:
	Assign a variable to an `mkword_hole' rval, for a construction
	unification of a direct arg functor with a free argument.

	Reassign the variable to an `mkword' rval when the argument becomes
	bound in a later unification.

compiler/code_info.m:
compiler/var_locn.m:
	Add a predicate to reassign a variable from a `mkword_hole' expression
	to a `mkword' expression.

compiler/llds_out_data.m:
	Write out `mkword_hole' values as a tagged null pointer in C code.

compiler/call_gen.m:
compiler/code_util.m:
compiler/dupelim.m:
compiler/dupproc.m:
compiler/exprn_aux.m:
compiler/global_data.m:
compiler/jumpopt.m:
compiler/livemap.m:
compiler/llds_to_x86_64.m:
compiler/middle_rec.m:
compiler/opt_debug.m:
compiler/opt_util.m:
compiler/peephole.m:
compiler/stack_layout.m:
	Conform to addition of `mkword_hole'.

tests/hard_coded/Mmakefile:
tests/hard_coded/direct_arg_partial_inst.exp:
tests/hard_coded/direct_arg_partial_inst.m:
tests/hard_coded/direct_arg_partial_inst2.exp:
tests/hard_coded/direct_arg_partial_inst2.m:
	Add test cases.