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

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

- File doc comments that aren't marked with \file, so they're attached to
  the next declaration, which is usually "namespace llvm {".
- Class doc comments placed before the namespace rather than before the
  class.
- Code comments before the namespace that (in my opinion) shouldn't be
  extracted by doxygen at all.

This commit fixes these comments. The generated doxygen documentation now
has proper docs for several classes and files, and the docs for the llvm
and llvm::detail namespaces are now empty.

Reviewed By: thakis, mizvekov

Differential Revision: https://reviews.llvm.org/D96736