[PATCH] emit a trap for buffer overflow in placement new

[Martin Sebor]

The -Wplacement-new option warns for buffer overflow in placement
new expressions with objects of constant sizes, but because it's
implemented completely in the C++ front end it misses the more
interesting non-constant sizes.

The attached patch instruments both forms of operator placement
new to emit a trap when __builtin_object_size() determines that
the pointer points to an object less than the specified number
of bytes.  This is done only when _FORTIFY_SOURCE is defined
to a non-zero value.  This makes it possible to prevent buffer
overflow in most of the same cases as in built-ins like strcpy,
though without warnings when the size is nor a C++ constant
integer expression.

On x86_64-linux it passes testing with no apparent regressions.
Can anyone think of problems with this solution?  If not, given
its simplicity, would it be appropriate even at this stage?

Martin

PS I added the tests to the G++ test suite rather than to
libstdc++ because the latter doesn't understand the DejaGnu
scan-tree-dump directive (looks like it's missing an import
for the .exp file that defines it).
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