[libclang] Expose completion result kind in `CXCompletionResult`

This allows clients of libclang to check whether a completion result is a keyword. Previously, keywords had `CursorKind == CXCursor_NotImplemented` and it wasn't trivial to distinguish a keyword from a pattern.

This change moves `CodeCompletionResult::ResultKind` to `clang-c` under a new name `CXCompletionResultKind`. It also tweaks `c-index-test` to print the result kind instead of `NotImplemented`, and adjusts the tests for the new output.

rdar://91852088

Differential Revision: https://reviews.llvm.org/D136844