[AST] Attach comment in `/** doc */ typedef struct A {} B` to B as well as A.

Summary:
Semantically they're the same thing, and it's important when the underlying
struct is anonymous.

There doesn't seem to be a problem attaching the same comment to multiple things
as it already happens with `/** doc */ int a, b;`

This affects an Index test but the results look better (name present, USR points
to the typedef).

Fixes https://github.com/clangd/clangd/issues/189

Reviewers: kadircet, lh123

Subscribers: ilya-biryukov, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70203