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.

1 files changed, 12 insertions, 0 deletions

diff --git a/test/files/neg/t8450.scala b/test/files/neg/t8450.scala
new file mode 100644
index 0000000000..f20ed2bc31
--- /dev/null
+++ b/test/files/neg/t8450.scala
@@ -0,0 +1,12 @@
+trait Foo
+
+class WarnWidening {
+  implicit class FooDouble(d: Double) { def foo = new Foo {} }
+  def elapsed: Foo = (System.nanoTime - 100L).foo
+}
+
+class NoWarnWidening {
+  implicit class FooLong(l: Long) { def foo = new Foo {} }
+  implicit class FooDouble(d: Double) { def foo = new Foo {} }
+  def elapsed: Foo = (System.nanoTime - 100L).foo
+}