[ASTUnit] Fix a regression in cached completions

Summary:
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).

Fixes https://bugs.llvm.org/show_bug.cgi?id=42646

Reviewers: kadircet, sammccall

Reviewed By: kadircet, sammccall

Subscribers: arphaman, nik, sammccall, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64918