Fix missed exception spec checks and crashes

Delayed exception specification checking for defaulted members and virtual
destructors are both susceptible to mutation during iteration so we need to
swap and process the worklists.

This resolves both accepts-invalid and rejects-valid issues and moreover fixes
potential invalid memory access as the contents of the vectors change during
iteration and recursive template instantiation.

Checking can be further delayed where parent classes aren't yet fully defined.
This patch adds two assertions at end of TU to ensure no specs are left
unchecked as was happenning before the fix, plus a test case from Marshall Clow
for the defaulted member crash extracted from the libcxx headers.

Reviewed by Richard Smith.
Check "late parsed" friend functions for redefinition

r177003 applied the late parsed template technique to friend functions
but omitted the corresponding check for redefinitions.

This patch adds the same check already in use for templates to the
new code path in order to diagnose and reject invalid redefinitions
that were being silently accepted.

Fixes PR17324.

Reviewed by Richard Smith.