Merge NamespaceSpecifierSet's AddNamespace and AddRecord as they are
essentially the same.
Be smarter about deciding to add a leading '::' to a
NestedNameSpecifier that replaces an existing specifier.
Allow CorrectTypo to replace CXXScopeSpecifiers that refer to classes.

Now that CorrectTypo knows how to correctly search classes for typo
correction candidates, there is no good reason to only replace an
existing CXXScopeSpecifier if it refers to a namespace. While the actual
enablement was a matter of changing a single comparison, the fallout
from enabling the functionality required a lot more code changes
(including my two previous commits).