Reinstate fix for PR7526, which was failing because, now that we

aren't dropping all exception specifications on destructors, the
exception specifications on implicitly-declared destructors were
detected as being wrong (which they were). 

Introduce logic to provide a proper exception-specification for
implicitly-declared destructors. This also fixes PR6972.

Note that the other implicitly-declared special member functions also
need to get exception-specifications. I'll deal with that in a
subsequent commit.

llvm-svn: 107385