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, 15 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/internal/TreeInfo.scala b/src/reflect/scala/reflect/internal/TreeInfo.scala
index 3e74b5d22d..fa4441e513 100644
--- a/src/reflect/scala/reflect/internal/TreeInfo.scala
+++ b/src/reflect/scala/reflect/internal/TreeInfo.scala
@@ -17,7 +17,7 @@ abstract class TreeInfo {
   val global: SymbolTable
 
   import global._
-  import definitions.{ isVarArgsList, isCastSymbol, ThrowableClass, TupleClass, MacroContextClass, MacroContextPrefixType }
+  import definitions.{ isTupleSymbol, isVarArgsList, isCastSymbol, ThrowableClass, TupleClass, MacroContextClass, MacroContextPrefixType }
 
   /* Does not seem to be used. Not sure what it does anyway.
   def isOwnerDefinition(tree: Tree): Boolean = tree match {
@@ -515,6 +515,20 @@ abstract class TreeInfo {
     case _        => false
   }
 
+  /**
+   * {{{
+   * //------------------------ => effectivePatternArity(args)
+   * case Extractor(a)          => 1
+   * case Extractor(a, b)       => 2
+   * case Extractor((a, b))     => 2
+   * case Extractor(a @ (b, c)) => 2
+   * }}}
+   */
+  def effectivePatternArity(args: List[Tree]): Int = (args.map(unbind) match {
+    case Apply(fun, xs) :: Nil if isTupleSymbol(fun.symbol) => xs
+    case xs                                                 => xs
+  }).length
+
 
   // used in the symbols for labeldefs and valdefs emitted by the pattern matcher
   // tailcalls, cps,... use this flag combination to detect translated matches