Introduce four new code-completion hooks for C++:

Introduce four new code-completion hooks for C++:

  - after "using", show anything that can be a nested-name-specifier.
  - after "using namespace", show any visible namespaces or namespace aliases
  - after "namespace", show any namespace definitions in the current scope
  - after "namespace identifier = ", show any visible namespaces or
    namespace aliases

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@82251 91177308-0d34-0410-b5e6-96231b3b80d8