gdb: Forward VALUE_LVAL when avoiding side effects for STRUCTOP_PTR

Assume that we have a C program like this:

  struct foo_type
  {
    int var;
  } foo;

  struct foo_type *foo_ptr = &foo;

  int
  main ()
  {
    return foo_ptr->var;
  }

Then GDB should be able to evaluate the following, however, it currently
does not:

  (gdb) start
  ...
  (gdb) whatis &(foo_ptr->var)
  Attempt to take address of value not located in memory.

The problem is that in EVAL_AVOID_SIDE_EFFECTS mode,
eval.c:evaluate_subexp_standard always returns a not_lval value as the
result for a STRUCTOP_PTR operation. As a consequence, the rest of
the code believes that one cannot take the address of the returned
value.

This patch fixes STRUCTOP_PTR handling so that the VALUE_LVAL
attribute for the returned value is properly initialized.  After this
change, the above session becomes:

  (gdb) start
  ...
  (gdb) whatis &(foo_ptr->var)
  type = int *

This commit is largely the same as commit 2520f728b710 (Forward
VALUE_LVAL when avoiding side effects for STRUCTOP_STRUCT) but applied
to STRUCTOP_PTR rather than STRUCTOP_STRUCT.  Both of these commits are
building on top of commit ac1ca910d74d (Fixes for PR exp/15364).

gdb/ChangeLog:

	* eval.c (evaluate_subexp_standard): If EVAL_AVOID_SIDE_EFFECTS
	mode, forward the VALUE_LVAL attribute to the returned value in
	the STRUCTOP_PTR case.

gdb/testsuite/ChangeLog:

	* gdb.base/whatis.c: Extend the test case.
	* gdb.base/whatis.exp: Add additional tests.

1 files changed, 1 insertions, 1 deletions

diff --git a/gdb/eval.c b/gdb/eval.c
index 3f8ca66..de1c663 100644
--- a/gdb/eval.c
+++ b/gdb/eval.c
@@ -1917,7 +1917,7 @@ evaluate_subexp_standard (struct type *expect_type,
       arg3 = value_struct_elt (&arg1, NULL, &exp->elts[pc + 2].string,
 			       NULL, "structure pointer");
       if (noside == EVAL_AVOID_SIDE_EFFECTS)
-	arg3 = value_zero (value_type (arg3), not_lval);
+	arg3 = value_zero (value_type (arg3), VALUE_LVAL (arg3));
       return arg3;
 
     case STRUCTOP_MEMBER: