[clangd] Show lambda signature for lambda autocompletions

The original bug report can be found
[here](https://github.com/clangd/clangd/issues/85)

Given the following code:

```c++
void function() {
  auto Lambda = [](int a, double &b) {return 1.f;};
  La^
}
```

Triggering the completion at `^` would show `(lambda)` before this patch
and would show signature `(int a, double &b) const`, build a snippet etc
with this patch.

Reviewers: sammccall

Reviewed by: sammccall

Differential revision: https://reviews.llvm.org/D70445