Teach typo correction to look inside of classes like it does namespaces.

Unlike with namespaces, searching inside of classes requires also
checking the access to correction candidates (i.e. don't suggest a
correction to a private class member for a correction occurring outside
that class and its methods or friends).

Included is a small (one line) fix for a bug, that was uncovered while
cleaning up the unit tests, where the decls from a TypoCorrection candidate
were preserved in new TypoCorrection candidates that are derived (copied)
from the old TypoCorrection--notably when creating a new candidate by
changing the NestedNameSpecifier associated with the base idenitifer.

llvm-svn: 191449