SI-6675 Avoid spurious warning about pattern bind arity.

In 692372ce, we added a warning (under -Xlint) when binding
a `TupleN` in to a single pattern binder, which wasn't allowed
before 2.10.0, and more often than not represents a bug.

However, that warning overstretched, and warned even when
using a Tuple Pattern to bind to the elements of such a value.

This commit checks for this case, and avoids the spurious warnings.

A new test case is added for this case to go with the existing
test for SI-6675:

    $ ./tools/partest-ack 6675
    % tests-with-matching-paths      ...  3
    % tests-with-matching-code       ...  2
    # 3 tests to run.
    test/partest --show-diff --show-log                  \
      test/files/neg/t6675-old-patmat.scala              \
      test/files/neg/t6675.scala                         \
      test/files/pos/t6675.scala                         \
      ""

    Testing individual files
    testing: [...]/files/pos/t6675.scala                                  [  OK  ]

    Testing individual files
    testing: [...]/files/neg/t6675-old-patmat.scala                       [  OK  ]
    testing: [...]/files/neg/t6675.scala                                  [  OK  ]
    All of 3 tests were successful (elapsed time: 00:00:03)

1 files changed, 4 insertions, 0 deletions

diff --git a/bincompat-forward.whitelist.conf b/bincompat-forward.whitelist.conf
index c59d0a7dec..82f094bed7 100644
--- a/bincompat-forward.whitelist.conf
+++ b/bincompat-forward.whitelist.conf
@@ -422,6 +422,10 @@ filter {
         {
             matchName="scala.reflect.internal.Types#TypeVar.setInst"
             problemName=IncompatibleResultTypeProblem
+        },
+        {
+             matchName="scala.reflect.internal.TreeInfo.effectivePatternArity"
+             problemName=MissingMethodProblem
         }
     ]
 }