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, 24 insertions, 0 deletions

diff --git a/test/files/neg/t5691.check b/test/files/neg/t5691.check
new file mode 100644
index 0000000000..a51ca98a10
--- /dev/null
+++ b/test/files/neg/t5691.check
@@ -0,0 +1,24 @@
+t5691.scala:7: warning: type parameter D defined in method foobar shadows trait D defined in class B. You may want to rename your type parameter, or possibly remove it.
+  def foobar[D](in: D) = in.toString
+             ^
+t5691.scala:10: warning: type parameter D defined in type MySeq shadows trait D defined in class B. You may want to rename your type parameter, or possibly remove it.
+  type MySeq[D] = Seq[D]
+             ^
+t5691.scala:15: warning: type parameter T defined in method bar shadows type T defined in class Foo. You may want to rename your type parameter, or possibly remove it.
+    def bar[T](w: T) = w.toString
+            ^
+t5691.scala:13: warning: type parameter T defined in class Foo shadows type T defined in class B. You may want to rename your type parameter, or possibly remove it.
+  class Foo[T](t: T) {
+            ^
+t5691.scala:19: warning: type parameter List defined in type M shadows type List defined in package object scala. You may want to rename your type parameter, or possibly remove it.
+  class C[M[List[_]]]
+            ^
+t5691.scala:20: warning: type parameter List defined in type M shadows type List defined in package object scala. You may want to rename your type parameter, or possibly remove it.
+  type E[M[List[_]]] = Int
+           ^
+t5691.scala:21: warning: type parameter List defined in type M shadows type List defined in package object scala. You may want to rename your type parameter, or possibly remove it.
+  def foo[N[M[List[_]]]] = ???
+              ^
+error: No warnings can be incurred under -Xfatal-warnings.
+7 warnings found
+one error found