Allow pointer arithmetic with integer references

Considering these variables:
int i = 3;
int &iref = i;

It's not possible to do any pointer arithmetic with iref:
(gdb) p &i+iref
Argument to arithmetic operation not a number or boolean.

So this adds checks for references to integers in pointer arithmetic.

gdb/ChangeLog:

2020-04-01  Hannes Domani  <ssbssa@yahoo.de>

	PR gdb/24789
	* eval.c (is_integral_or_integral_reference): New function.
	(evaluate_subexp_standard): Allow integer references in
	pointer arithmetic.

gdb/testsuite/ChangeLog:

2020-04-01  Hannes Domani  <ssbssa@yahoo.de>

	PR gdb/24789
	* gdb.cp/misc.cc: Add integer reference variable.
	* gdb.cp/misc.exp: Add test.