Switch c-index-test from clang_codeComplete() over to

clang_codeCompleteAt(). This uncovered a few issues with the latter:

  - ASTUnit wasn't saving/restoring diagnostic state appropriately between
    reparses and code completions.
  - "Overload" completions weren't being passed through to the client



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@116241 91177308-0d34-0410-b5e6-96231b3b80d8