Improve MI -var-info-path-expression for nested struct/union case.

https://sourceware.org/ml/gdb-patches/2014-05/msg00383.html

The MI command -var-info-path-expression currently does not handle
non-anonymous structs / unions nested within other structs / unions,
it will skip parts of the expression.  Consider this example:

  ## START EXAMPLE ##
  $ cat ex.c
  #include <string.h>

  int
  main ()
  {
    struct s1
    {
      int a;
    };

    struct ss
    {
      struct s1 x;
    };

    struct ss an_ss;
    memset (&an_ss, 0, sizeof (an_ss));
    return 0;
  }
  $ gcc -g -o ex.x ex.c
  $ gdb ex.x
  (gdb) break 18
  Breakpoint 1 at 0x80483ba: file ex.c, line 18.
  (gdb) run
  Starting program: /home/user/ex.x

  Breakpoint 1, main () at ex.c:18
  18	  return 0;
  (gdb) interpreter-exec mi "-var-create an_ss * an_ss"
  (gdb) interpreter-exec mi "-var-list-children an_ss"
  ^done,numchild="1",children=[child={name="an_ss.x",exp="x",numchild="1",type="struct s1",thread-id="1"}],has_more="0"
  (gdb) interpreter-exec mi "-var-list-children an_ss.x"
  ^done,numchild="1",children=[child={name="an_ss.x.a",exp="a",numchild="0",type="int",thread-id="1"}],has_more="0"
  (gdb) interpreter-exec mi "-var-list-children an_ss.x.a"
  ^done,numchild="0",has_more="0"
  (gdb) interpreter-exec mi "-var-info-path-expression an_ss.x.a"
  ^done,path_expr="(an_ss).a"
  (gdb) print (an_ss).a
  There is no member named a.
  ## END EXAMPLE ##

Notice that the path expression returned is wrong, and as a result
the print command fails.

This patch adds a new method to the varobj_ops structure called
is_path_expr_parent, to allow language specific control over finding
the parent varobj, the current logic becomes the C/C++ version and is
extended to handle the nested cases.  No other language currently uses
this code, so all other languages just get a default method.

With this patch, the above example now finishes like this:

  ## START EXAMPLE ##
  $ gdb ex.x
  (gdb) break 18
  Breakpoint 1 at 0x80483ba: file ex.c, line 18.
  (gdb) run
  Starting program: /home/user/ex.x

  Breakpoint 1, main () at ex.c:18
  18	  return 0;
  (gdb) interpreter-exec mi "-var-list-children an_ss"
  ^done,numchild="1",children=[child={name="an_ss.x",exp="x",numchild="1",type="struct s1",thread-id="1"}],has_more="0"
  (gdb) interpreter-exec mi "-var-list-children an_ss.x"
  ^done,numchild="1",children=[child={name="an_ss.x.a",exp="a",numchild="0",type="int",thread-id="1"}],has_more="0"
  (gdb) interpreter-exec mi "-var-list-children an_ss.x.a"
  ^done,numchild="0",has_more="0"
  (gdb) interpreter-exec mi "-var-info-path-expression an_ss.x.a"
  ^done,path_expr="((an_ss).x).a"
  (gdb) print ((an_ss).x).a
  $1 = 0
  ## END EXAMPLE ##

Notice that the path expression is now correct, and the print is a
success.

gdb/ChangeLog:

	* ada-varobj.c (ada_varobj_ops): Fill in is_path_expr_parent
	field.
	* c-varobj.c (c_is_path_expr_parent): New function, moved core
	from varobj.c, with additional checks.
	(c_varobj_ops): Fill in is_path_expr_parent field.
	(cplus_varobj_ops): Fill in is_path_expr_parent field.
	* jv-varobj.c (java_varobj_ops): Fill in is_path_expr_parent
	field.
	* varobj.c (is_path_expr_parent): Call is_path_expr_parent varobj
	ops method.
	(varobj_default_is_path_expr_parent): New function.
	* varobj.h (lang_varobj_ops): Add is_path_expr_parent field.
	(varobj_default_is_path_expr_parent): Declare new function.

gdb/testsuite/ChangeLog:

	* gdb.mi/var-cmd.c (do_nested_struct_union_tests): New function
	setting up test structures.
	(main): Call new test function.
	* gdb.mi/mi2-var-child.exp: Create additional breakpoint in new
	test function, continue into test function and walk test
	structures.

gdb/ada-varobj.c

@@ -1026,5 +1026,6 @@ const struct lang_varobj_ops ada_varobj_ops =
   ada_type_of_child,
   ada_value_of_variable,
   ada_value_is_changeable_p,
-  ada_value_has_mutated
+  ada_value_has_mutated,
+  varobj_default_is_path_expr_parent
 };