Logical short circuiting with argument lists

When evaluating Fortran expressions such as the following:

	print truth_table(1,1) .OR. truth_table(2,1)

where truth_table(1,1) evaluates to true, the debugger would report that
it could not perform substring operations on this type. This patch
addresses this issue.

Investigation revealed that EVAL_SKIP was not being handled correctly
for all types serviced by the OP_F77_UNDETERMINED_ARGLIST case in
evaluate_subexp_standard. While skipping an undetermined argument list
the type is resolved to be an integer (as this is what evaluate_subexp
returns when skipping) and so it was not possible to delegate to the
appropriate case (e.g. array, function call).

The solution implemented here updates OP_VAR_VALUE to return correct
type information when skipping. This way OP_F77_UNDETERMINED_ARGLIST
can delegate the skipping to the appropriate case or routine, which
should know how to skip/evaluate the type in question.

koenig.exp was updated to include a testcase which exercises the
modified skip logic in OP_VAR_VALUE, as it falls through from
OP_ADL_FUNC.

This patch has been tested for regressions with GCC 7.3 on aarch64,
ppc64le and x86_64.

gdb/ChangeLog:

	* eval.c (skip_undetermined_arglist): Skip argument list helper.
	(evaluate_subexp_standard): Return a dummy type when
	honoring EVAL_SKIP in OP_VAR_VALUE and handle skipping in the
	OP_F77_UNDETERMINED_ARGLIST case.
	* expression.h (enum noside): Update comment.

gdb/testsuite/ChangeLog:

	* gdb.cp/koenig.exp: Extend to test logical short circuiting.
	* gdb.fortran/short-circuit-argument-list.exp: New file.
	* gdb.fortran/short-circuit-argument-list.f90: New test.

gdb/expression.h

@@ -118,7 +118,9 @@ extern int parse_completion;
 enum noside
   {
     EVAL_NORMAL,
-    EVAL_SKIP,			/* Only effect is to increment pos.  */
+    EVAL_SKIP,			/* Only effect is to increment pos.
+				   Return type information where
+				   possible.  */
     EVAL_AVOID_SIDE_EFFECTS	/* Don't modify any variables or
 				   call any functions.  The value
 				   returned will have the correct