Implement support for cached code completions for

nested-name-specifiers. Also includes fixes to the generation of
nested-name-specifier result in the non-cached case; we were producing
lame results for namespaces and namespace aliases, which (1) didn't
always have nested-name-specifiers when we want them, and (2) did not
have the necessary "::" as part of the completion.

llvm-svn: 111203