[C++ DR 2336] virtual dtors, exception specs & abstract classes

[Nathan Sidwell]

This patch implements dr2336, a fix to dr1658

dr1658 made virtual bases of abstract classes ignored for synthesize 
cdtors -- there are no complete objects of such type, so the vbases will 
never be cdtored by such cdtors.

Except when virtual dtors are in play, such a dtor could override a 
virtual dtor in a virtual base, and thus must not have a stricter 
exception specification.  DR2336 addresses that.

I had nearly anticipated this resolution, but I was also looking into 
virtual bases in other cases, and ignoring access in different other 
cases.   This implements the DR2336 wording (it does not distinguish 
between virtual and non-virtual dtors of virtual bases).

booted on x6_64-linux, committing to trunk.

nathan
-- 
Nathan Sidwell
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