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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107385 91177308-0d34-0410-b5e6-96231b3b80d8