[committed] C++ front end patch for too large arrays

[James E Wilson <wilson at specifix dot com>]

This partially fixes the problem described in PR 23426.  An array larger
than half the address space generates an ICE in tree_int_cst_low.  With
this patch, we now get an error.  Unfortunately, we get two errors, and
the second one doesn't make any sense, so there is still a problem that
needs to be fixed, but at least we don't get the ICE anymore.

This was tested with an x86_64-linux bootstrap and make check.  There
were no regressions.  Mark Mitchell OKed the patch in the PR.  This is
the same as a patch added to the C front end last week.

I have added the patch to mainline and to the gcc-4.0.x branch.  I
needed to backport a small patch by Roger Sayle from mainline for the
gcc-4.0.x branch so it is a slightly different patch.
-- 
Jim Wilson, GNU Tools Support, http://www.specifix.com

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