SI-6891 Fix value class + tailrec crasher.

rhs.substituteSymbols(old, new) leaves us with:

    def loop#12225(x#12226: A#15491): scala#21.this.Unit#1615 =
      loop#12225(x#12226)

In which the TermSymbol x#12226 has a stale info, pointing at
the A#7274, the class type parameter, rather than A#15491,
the corresponding type parameter of the synthetic backing
method.

I've improved `TreeSymSubstituter` to substitute not
only `Tree#{tpe, symbol}`, but also `DefTree#sym.info`.

The `pos` test that triggered the new code path are
listed here: https://gist.github.com/4575687

AFAICS, no special treatment of Function, Return, or Import
is needed in TreeSymSubstutor.

2 files changed, 7 insertions, 0 deletions

diff --git a/test/pending/pos/t6891.flags b/test/files/pos/t6891.flags
index fe048006aa..fe048006aa 100644
--- a/test/pending/pos/t6891.flags
+++ b/test/files/pos/t6891.flags
diff --git a/test/pending/pos/t6891.scala b/test/files/pos/t6891.scala
index bf79c2d293..bed2d0d777 100644
--- a/test/pending/pos/t6891.scala
+++ b/test/files/pos/t6891.scala
@@ -9,6 +9,13 @@ object O {
       @annotation.tailrec def loop(x: value.type): Unit = loop(x)
       ()
     }
+
+    def beppy[C](c: => C) = {
+      () => c
+      @annotation.tailrec def loop(x: value.type): Unit = loop(x)
+        () => c
+      ()
+    }
   }
   // uncaught exception during compilation: Types$TypeError("type mismatch;
   //  found   : A(in method bippy$extension)