PATCH: option to silence warnings about offsetof and non-PODs

[Matt Austern <austern at apple dot com>]

Some internal users at Apple have complained about the warning message 
for applying offsetof to non-PODs.  I've seen similar complaints from 
people outside Apple.

The warning message is of course correct, and one could even argue that 
it should be a hard error.  Applying offsetof to a non-POD is 
undefined.  However, there are also a lot of cases where "POD" is too 
strict a concept, where the offset of a data member is well defined and 
the restriction is purely artificial.  (I've raised this issue with the 
C++ standards committee, and there's general agreement that "POD" was 
chosen mostly for simplicity rather than because it's essential to the 
notion of offsets.)  Applying offsetof to simple non-POD types, such as 
a simple struct with a constructor, is common.  Users who are aware 
that they're writing nonportable code, and who have deliberately chosen 
to ignore the warning about it, should be able to do so.

This patch provides a switch, -W(no-)invalid-offsetof, to control the 
warning.  The warning is on by default.  The only way to turn it off is 
to use this switch explicitly; it is not controlled by -fpermissive or 
by any other switch.

Documentation, test cases, and changelog entries included.  
Bootstrapped and tested on i686-linux.

OK to commit to mainline?

			--Matt

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