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, 11 insertions, 0 deletions

diff --git a/test/files/neg/t8736-c.check b/test/files/neg/t8736-c.check
new file mode 100644
index 0000000000..06b2228543
--- /dev/null
+++ b/test/files/neg/t8736-c.check
@@ -0,0 +1,11 @@
+t8736-c.scala:4: warning: higher-kinded type should be enabled
+by making the implicit value scala.language.higherKinds visible.
+This can be achieved by adding the import clause 'import scala.language.higherKinds'
+or by setting the compiler option -language:higherKinds.
+See the Scala docs for value scala.language.higherKinds for a discussion
+why the feature should be explicitly enabled.
+  def hk[M[_]] = ???
+         ^
+error: No warnings can be incurred under -Xfatal-warnings.
+one warning found
+one error found