SI-5691 lint warning when a type parameter shadows an existing type.

This adds a new lint warning for when a class/method/type-member's
type parameter shadows an existing type: `-Xlint:type-parameter-shadow`.

It excludes type parameters of synthetic methods (the user can't
rename or remove those anyway), otherwise, for example, every case class
triggers the warning.

Also fixes a test that contained wrong java sources (that didn't even
compile...), discovered thanks to the warning.

---
This kind of errors shows up every now and then on the mailing-list, on
stackoverflow, etc. so maybe a warning would be useful.
I was afraid this would yield too many warnings for libraries that are
heavy on type parameters, but no: running this on scalaz and shapeless
HEAD (`v7.1.0-RC1-41-g1cc0a96` and `v2.0.0-M1-225-g78426a0` respectively)
yields 44 warnings. None of them are false positives; they usually come
from:
 - scalaz loving using `A` as type parameter, even several levels deep
   of parametrized classes/methods
 - or calling a type parameter that will hold a map `Map`, or similar,
   thus shadowing an existing type

1 files changed, 1 insertions, 1 deletions

diff --git a/test/files/neg/t6675b.scala b/test/files/neg/t6675b.scala
index c86c9c3955..da27e1b91f 100644
--- a/test/files/neg/t6675b.scala
+++ b/test/files/neg/t6675b.scala
@@ -13,7 +13,7 @@ object NativelyTwo {
 }
 
 
-class A {
+class E {
   def f1 = (Left((0, 0)): Either[(Int, Int), (Int, Int)]) match { case LeftOrRight(a) => a  }          // warn
   def f2 = (Left((0, 0)): Either[(Int, Int), (Int, Int)]) match { case LeftOrRight((a, b)) => a  }     // no warn
   def f3 = (Left((0, 0)): Either[(Int, Int), (Int, Int)]) match { case LeftOrRight((a, b, c)) => a  }  // fail