[clangd] Rearrange type, returntype and parameters in hover card

Summary:
Moves type/returntype into its own line as it is more readable in cases
where the type is long.

Also gives parameter lists a heading, `Parameters:` to make them stand out.

Leaves the `right arrow` instead of `Returns: ` before Return Type to make
output more symmetric.

```
function foo

Returns: ret_type
Parameters:
- int x
```

vs

```
function foo

🡺 ret_type
Parameters:
- int x
```

Reviewers: sammccall, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72623