Added error message

[Richard Kenner]

What was the purpose of adding an error message in layout_type when an
array's size cannot be represented in size_t?

We should give an error if the size of an *object* can't be
representable, but why a type?  Note that you don't check for RECORD_TYPE,
just ARRAY_TYPE.

It's a fairly common idiom in Ada to make an array type whose bounds
are all integers to represent "all memory".  If it's an array of bytes
that's OK, but not if it's an array of anything wider.  So this error
causes problems there.

But I don't understand its purpose at all.