Allow typo correction to try removing nested name specifiers.

The removal is tried by retrying the failed lookup of a correction
candidate with either the MemberContext or SS (CXXScopeSpecifier) or
both set to NULL if they weren't already. If the candidate identifier
is then looked up successfully, make a note in the candidate that the
SourceRange should include any existing nested name specifier even if
the candidate isn't adding a different one (i.e. the candidate has a
NULL NestedNameSpecifier).

Also tweak the diagnostic messages to differentiate between a suggestion
that just replaces the identifer but leaves the existing nested name
specifier intact and one that replaces the entire qualified identifier,
in cases where the suggested replacement is unqualified.