[clangd] Fix a regression of not showing documentation from forward declarations.

Summary:
There is a regression from https://reviews.llvm.org/D68467. Unlike class
forward declarations, function ducomentation is written in the declaration in
headers, the function definition doesn't contain any documentation, cases like:

```
foo.h
// this is foo.
void foo();
foo.cc

void foo() {}
```
we should still show documentation from the foo declaration.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69961