SI-8450 no "implicit numeric widening" in silent mode

Before this fix, we get a second, spurious warning.

```
t8450.scala:5: warning: implicit numeric widening
  def elapsed: Foo = (System.nanoTime - 100L).foo
                                      ^
t8450.scala:11: warning: implicit numeric widening
  def elapsed: Foo = (System.nanoTime - 100L).foo
                                      ^
error: No warnings can be incurred under -Xfatal-warnings.
two warnings found
one error found
```

By respecting silent contexts, we now get only one.

I sneakily fixed similar occurrences without adding a test.
Merge pull request #3658 from adriaanm/t8450

SI-8450 no "implicit numeric widening" in silent mode