[docs] Fix doxygen comments wrongly attached to the clang namespace

Looking at the Doxygen-generated documentation for the clang namespace
currently shows several random comments from different parts of the
codebase. These are caused by:

- File doc comments that aren't marked with \file, so they're attached to
  the next declaration, which is usually "namespace clang {".
- Class doc comments placed before the namespace rather than before the
  class.

This commit fixes these comments. The generated doxygen documentation now
has proper docs for several classes and files, and the docs for the clang
namespace is now empty.

Differential Revision: https://reviews.llvm.org/D96738