SI-8736 Restore -language to former glory

MultiChoice allows -language to work like -Xlint.

The bug as described was that the setting value was set instead of updated
(++=) with additional flags.

The unreported bug was that `_` no longer set all settings.

The corrected behavior is that "contains" means "it was enabled, or
_ was set and it was not disabled explicitly."

That is the behavior of `-Xlint` but with a different mechanism,
since each lint option is a Setting.

A semantic difference is that -Xlint enables all the lint options,
but -language does not enable all the language features. `scalac -X` does
not explain this additional behavior of the `-Xlint` flag.

Also worth noting that `scalac -feature -language unused.scala` failed
in 2.11.1 but succeeds silently now.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/files/pos/t8736-b.scala b/test/files/pos/t8736-b.scala
new file mode 100644
index 0000000000..903292d232
--- /dev/null
+++ b/test/files/pos/t8736-b.scala
@@ -0,0 +1,7 @@
+// scalac: -feature -language:_ -Xfatal-warnings
+// showing that all are set
+class X {
+  def hk[M[_]] = ???
+
+  implicit def imp(x: X): Int = x.hashCode
+}