Added error message (for too-large array)

[Richard Kenner]

    OK.  You're saying that an array with an unrepresentable size is valid in
    Ada, so my assumption that it would be invalid for all languages was
    incorrect.

"Unrepresentable" here means "... in the GCC tree".  In Ada, all
constants are to be interpreted in infinite precision and the size of
types are likewise.  So it's improper to say that some type is too large.

    I suppose I'll move the test into layout_decl, since you seem to agree
    that creating an object of such a type is invalid.

Right.  Even in Ada, an implementation is allowed (indeed expected) to
limit the amount of storage that can be allocated to an object.