Align required/found pairs in diagnostics

javac has some diagnostics that list 'required' and 'found' pairs, e.g. of type arguments. It's slightly easier to scan for the differences between those two lists if they're horizontally aligned, but the diagnostics aren't consistent about using horizontal alignment.

Before:

```
    required: {2}\n\
    found: {3}\n\
```

After:

```
    required: {2}\n\
    found:    {3}\n\
```