Improve typo correction involving nested name specifiers.

Snooping in other namespaces when the identifier being corrected is
already qualified (i.e. a valid CXXScopeSpec is passed to CorrectTypo)
and ranking synthesized namespace qualifiers relative to the existing
qualifier is now performed. Support for disambiguating the string
representation of synthesized namespace qualifers has also been added
(the change to test/Parser/cxx-using-directive.cpp is an example of an
ambiguous relative qualifier).

llvm-svn: 150622