[backport] Refactor the ClosureOptimizer, run ProdCons only once per method

Refactor and clean up the ClosureOptimzier. The goal of this change
is to run the ProdCons analyzer only once per method, instead of
once per closure instantiation.

Bootstrapping scala with `-Yopt-inline-heuristics:everything` revealed
that ProdCons can take a very long time on large methods, for example

```
[quick.compiler] scala/tools/nsc/backend/jvm/BCodeBodyBuilder$PlainBodyBuilder#genArithmeticOp - analysis: 1 spans, 17755ms
[quick.compiler] scala/tools/nsc/typechecker/SuperAccessors$SuperAccTransformer#transform - analysis: 1 spans, 28024ms
[quick.compiler] scala/tools/nsc/backend/jvm/BCodeBodyBuilder$PlainBodyBuilder#genInvokeDynamicLambda - analysis: 1 spans, 22100ms
```

With this change and enough time and space (-Xmx6000m), bootstrapping
scala succeeds in this test mode.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BackendReporting.scala b/src/compiler/scala/tools/nsc/backend/jvm/BackendReporting.scala
index 4eb24d13e3..b41d0de92f 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BackendReporting.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BackendReporting.scala
@@ -255,7 +255,7 @@ object BackendReporting {
    * Used in `rewriteClosureApplyInvocations` when a closure apply callsite cannot be rewritten
    * to the closure body method.
    */
-  trait RewriteClosureApplyToClosureBodyFailed extends OptimizerWarning {
+  sealed trait RewriteClosureApplyToClosureBodyFailed extends OptimizerWarning {
     def pos: Position
 
     override def emitWarning(settings: ScalaSettings): Boolean = this match {