[Committed] Only allow INTEGER_CSTs of integral/pointer types.

[Jason Merrill]

Jason Merrill wrote:
> Roger Sayle wrote:
>> Jason, were there any objections to this approach?  Did it bootstrap
>> and regression test without problems?
> 
> Nobody responded to my RFC.  I'm pretty sure it tested without problems, 
> but I'll test it again with the current tree.

Yep, still no regressions on x86_64-pc-linux-gnu.  If it seems 
reasonable to you, I'll go ahead and check it in.

Jason