swift234

- Reimplement FailureDiagnosis::diagnoseGeneralMemberFailure in terms of
  performMemberLookup, eliminating a ton of duplicated logic, but keeping the
  same general behavior.

- Now that r30787 landed, we can have diagnoseGeneralMemberFailure inform
  clients when a member lookup fails due to referencing a candidate decl of
  ErrorType (i.e, it is already invalid somehow).  When this happens, there is
  no reason to diagnose a problem, because the original issue has been diagnosed
  and anything we produce now is just garbage.

The second point cleans up a bunch of bogus diagnostics in the testsuite, which are
*actually* due to upstream error that are already diagnosed.



Swift SVN r30789