[clangd] Add parameter and return type information to completion results

Summary:
This patch adds information about the parameters and return types of completion
candidates.
Previously, for the following code:
```
struct S {
  int func(int a, double b) const;
};
```
the completer would only return the label of the candidate `func`.
Now it will also return the return type `int` and will format the label for the
candidate as `func(int a, double b) const`.

Reviewers: bkramer, ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D34033

llvm-svn: 304980