[ASTUnit] Fix a regression in cached completions
After r345152 cached completions started adding namespaces after
nested name specifiers, e.g. in some_name::^
The CCC_Symbol indicates the completed item cannot be a namespace (it is
described as being "a type, a function or a variable" in the comments).
Therefore, 'nested specifier' completions should only be added from cache
when the context is CCC_SymbolOrNewName (which roughly seems to indicate
that a nested name specifier is allowed).
Reviewers: kadircet, sammccall
Reviewed By: kadircet, sammccall
Subscribers: arphaman, nik, sammccall, cfe-commits
Differential Revision: https://reviews.llvm.org/D64918