Minor tweaks for code-completion:

Minor tweaks for code-completion:
  - Filter out unnamed declarations
  - Filter out declarations whose names are reserved for the
  implementation (e.g., __bar, _Foo)
  - Place OVERLOAD: or COMPLETION: at the beginning of each
  code-completion result, so we can easily separate them from other
  compilation results.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@83680 91177308-0d34-0410-b5e6-96231b3b80d8