Eliminate make_cleanup_obstack_free, introduce auto_obstack

This commit eliminates make_cleanup_obstack_free, replacing it with a
new auto_obstack type that inherits obstack to add cdtors.

These changes in the parsers may not be obvious:

 -  obstack_init (&name_obstack);
 -  make_cleanup_obstack_free (&name_obstack);
 +  name_obstack.clear ();

Here, the 'name_obstack' variable is a global.  The change means that
the obstack's contents from a previous parse will stay around until
the next parsing starts.  I.e., memory won't be reclaimed until then.
I don't think that's a problem, these objects don't really grow much
at all.

The other option I tried was to add a separate type that is like
auto_obstack but manages an external obstack, just for those cases.  I
like the current approach better as that other approach adds more
boilerplate and yet another type to learn.

gdb/ChangeLog:
2017-06-27  Pedro Alves  <palves@redhat.com>

	* c-exp.y (name_obstack): Now an auto_obstack.
	(yylex): Use auto_obstack::clear.
	(c_parse): Use auto_obstack::clear instead of reinitializing and
	freeing the obstack.
	* c-lang.c (evaluate_subexp_c): Use auto_obstack.
	* d-exp.y (name_obstack): Now an auto_obstack.
	(yylex): Use auto_obstack::clear.
	(d_parse): Use auto_obstack::clear instead of reinitializing and
	freeing the obstack.
	* dwarf2loc.c (fetch_const_value_from_synthetic_pointer): Use
	auto_obstack.
	* dwarf2read.c (create_addrmap_from_index)
	(dwarf2_build_psymtabs_hard)
	(update_enumeration_type_from_children): Likewise.
	* gdb_obstack.h (auto_obstack): New type.
	* go-exp.y (name_obstack): Now an auto_obstack.
	(build_packaged_name): Use auto_obstack::clear.
	(go_parse): Use auto_obstack::clear instead of reinitializing and
	freeing the obstack.
	* linux-tdep.c (linux_make_mappings_corefile_notes): Use
	auto_obstack.
	* printcmd.c (printf_wide_c_string, ui_printf): Use auto_obstack.
	* rust-exp.y (work_obstack): Now an auto_obstack.
	(rust_parse, rust_lex_tests): Use auto_obstack::clear instead of
	reinitializing and freeing the obstack.
	* utils.c (do_obstack_free, make_cleanup_obstack_free): Delete.
	(host_char_to_target): Use auto_obstack.
	* utils.h (make_cleanup_obstack_free): Delete declaration.
	* valprint.c (generic_emit_char, generic_printstr): Use
	auto_obstack.

gdb/c-lang.c

@@ -570,15 +570,12 @@ evaluate_subexp_c (struct type *expect_type, struct expression *exp,
       {
 	int oplen, limit;
 	struct type *type;
-	struct obstack output;
-	struct cleanup *cleanup;
 	struct value *result;
 	c_string_type dest_type;
 	const char *dest_charset;
 	int satisfy_expected = 0;
 
-	obstack_init (&output);
-	cleanup = make_cleanup_obstack_free (&output);
+	auto_obstack output;
 
 	++*pos;
 	oplen = longest_to_int (exp->elts[*pos].longconst);
@@ -656,7 +653,6 @@ evaluate_subexp_c (struct type *expect_type, struct expression *exp,
 	      result = allocate_value (type);
 	    else
 	      result = value_cstring ("", 0, type);
-	    do_cleanups (cleanup);
 	    return result;
 	  }
 
@@ -702,7 +698,6 @@ evaluate_subexp_c (struct type *expect_type, struct expression *exp,
 				      obstack_object_size (&output),
 				      type);
 	  }
-	do_cleanups (cleanup);
 	return result;
       }
       break;