[NFC][Docs] Add missing Doxygen group comments for LLVM-C

The LLVM-C API is relatively small so we've previously added doxygen tags
so it's easier to navigate the LLVM-C web docs. Over the years, more
headers were added without proper doxygen tags, effectively hiding them
from the main LLVM-C doxygen page. This patch adds comments to headers
which did not have them.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D112474