SI-9286 Check subclass privates for "same type after erasure"

The overriding pairs cursor used to detect erased signature clashes
was turning a blind eye to any pair that contained a private method.

However, this could lead to a `VerifyError` or `IllegalAccessError`.

Checking against javac's behaviour in both directions:

```
% cat sandbox/Test.java
public abstract class Test {
  class C { int foo() { return 0; } }
  class D extends C { private <A> int foo() { return 1; } }
}

% javac sandbox/Test.java
sandbox/Test.java:3: error: name clash: <A>foo() in Test.D and foo() in Test.C have the same erasure, yet neither overrides the other
  class D extends C { private <A> int foo() { return 1; } }
                                      ^
  where A is a type-variable:
    A extends Object declared in method <A>foo()
1 error
```

```
% cat sandbox/Test.java
public abstract class Test {
  class C { private int foo() { return 0; } }
  class D extends C { <A> int foo() { return 1; } }
}

% javac sandbox/Test.java
%
```

This commit only the exludes private symbols from the superclass
from the checks by moving the test from `excludes` to `matches`.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/files/neg/t9286c.check b/test/files/neg/t9286c.check
new file mode 100644
index 0000000000..785cb3f937
--- /dev/null
+++ b/test/files/neg/t9286c.check
@@ -0,0 +1,7 @@
+t9286c.scala:8: error: name clash between defined and inherited member:
+def foo(m: M[_ >: String]): Int in trait T and
+private def foo(m: M[_ >: Any]): Int at line 8
+have same type after erasure: (m: M)Int
+    def foo(m: M[_ >: Any]) = 0 // Expected: "same type after erasure"
+        ^
+one error found