SI-6806 Add an @implicitAmbiguous annotation

Example usage:

    trait =!=[C, D]

    implicit def neq[E, F] : E =!= F = null

    @annotation.implicitAmbiguous("Could not prove ${J} =!= ${J}")
    implicit def neqAmbig1[G, H, J] : J =!= J = null
    implicit def neqAmbig2[I] : I =!= I = null

    implicitly[Int =!= Int]

Which gives the following error:

    implicit-ambiguous.scala:9: error: Could not prove Int =!= Int
      implicitly[Int =!= Int]
                ^

Better than what was previously given:

    implicit-ambiguous.scala:9: error: ambiguous implicit values:
     both method neqAmbig1 in object Test of type [G, H, J]=> Main.$anon.Test.=!=[J,J]
     and method neqAmbig2 in object Test of type [I]=> Main.$anon.Test.=!=[I,I]
     match expected type Main.$anon.Test.=!=[Int,Int]
      implicitly[Int =!= Int]
                ^

1 files changed, 1 insertions, 0 deletions

diff --git a/test/files/neg/implicit-ambiguous-invalid.flags b/test/files/neg/implicit-ambiguous-invalid.flags
new file mode 100644
index 0000000000..85d8eb2ba2
--- /dev/null
+++ b/test/files/neg/implicit-ambiguous-invalid.flags
@@ -0,0 +1 @@
+-Xfatal-warnings