[clangd] Don't use the optional "severity" when comparing Diagnostic.

Summary:
We use Diagnostic as a key to find the corresponding FixIt when we do
the "apply-fix", but the "severity" field could be omitted, in some cases,
the codeAction request sent from LSP clients (e.g. VScode) doesn't include the
`severity` field, which makes clangd fail to find the FixIt.

Test the following code in VScode, before the fix, no FixIt shown.

```
void main() {}
^~~~
```

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41280

llvm-svn: 321106