SI-8888 Avoid ClassFormatError under -Ydelambdafy:method

The pattern matcher phase (conceivably, among others) can generate
code that binds local `Ident`s symbolically, rather than according
to the lexical scope. This means that a lambda can capture more than
one local of the same name.

In the enclosed test case, this ends up creating the following
tree after delambdafy

    [[syntax trees at end of                delambdafy]] // delambday-patmat-path-dep.scala
    matchEnd4({
      case <synthetic> val x1: Object = (x2: Object);
      case5(){
        if (x1.$isInstanceOf[C]())
          {
            <synthetic> val x2#19598: C = (x1.$asInstanceOf[C](): C);
            matchEnd4({
              {
                (new resume$1(x2#19598, x2#19639): runtime.AbstractFunction0)
              };
              scala.runtime.BoxedUnit.UNIT
            })
          }
        else
          case6()
      };
      ...
    })
    ...
    <synthetic> class resume$1 extends AbstractFunction0 {
      <synthetic> var x2: C = null;
      <synthetic> var x2: C = null;
      ...
    }

After this commit, the var members of `resume$1` are given fresh
names, rather than directly using the name of the captured var:

    <synthetic> var x2$3: C = null;
    <synthetic> var x2$4: C = null;

2 files changed, 13 insertions, 0 deletions

diff --git a/test/files/run/t8888.flags b/test/files/run/t8888.flags
new file mode 100644
index 0000000000..48b438ddf8
--- /dev/null
+++ b/test/files/run/t8888.flags
@@ -0,0 +1 @@
+-Ydelambdafy:method
diff --git a/test/files/run/t8888.scala b/test/files/run/t8888.scala
new file mode 100644
index 0000000000..36cc1ddf3e
--- /dev/null
+++ b/test/files/run/t8888.scala
@@ -0,0 +1,12 @@
+class C {
+  final def resume: Unit = (this: Any) match {
+    case x : C => (x: Any) match {
+      case y : C =>
+        () => (x, y) // used to trigger a ClassFormatError under -Ydelambdafy:method
+    }
+  }
+}
+
+object Test extends App {
+  new C().resume
+}