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, 2 insertions, 2 deletions

diff --git a/test/files/neg/t4851/J2.java b/test/files/neg/t4851/J2.java
index 82954d9489..a90f48e269 100644
--- a/test/files/neg/t4851/J2.java
+++ b/test/files/neg/t4851/J2.java
@@ -1,11 +1,11 @@
 public class J2<T> {
   T x;
 
-  public <T> J(T x) {
+  public J2(T x) {
     this.x = x;
   }
 
   public String toString() {
     return "J2:" + x.getClass();
   }
-}
\ No newline at end of file
+}