Better backtrace in classloading for unreachable symbols

[WojciechMazur]

This change modifies the scope of reachability analysis results to limit the scope of passed data, that is never used after classloading finishes (unavailable symbols and referenced from maps). It now returns either an UnreachableSymbolsFound containing only information required to correctly point to usages of unreachble symbols, and Result containing the full information required for later stages.

This change also adds better backtraces for unreachable symbols. For example for this snippet:

object Test {
  def main(args: Array[String]): Unit = {
    class Foo(x: Long) extends java.sql.Time(x)
    trait Bar { self: java.sql.Types => }
    class Baz extends java.sql.Types with Bar
    0.until(10).foreach { _ =>
      println(Foo(1))
      println(Baz())
    }
  }
}

we would now get following output:

[error] Found 4 unreachable symbols
[error] Found unknown type java.sql.Time, referenced from:
[error]       type at Test$Foo$1(Test.scala:4)
[error]     method at Test$.main$$anonfun$1(Test.scala:8)
[error]     method at Test$$$Lambda$1.apply$mcVI$sp(Test.scala:9)
[error]     method at Test$.main(Test.scala:9)
[error]     method at Test.main(Test.scala:2)
[error] 
[error] Found unknown constructor java.sql.Types(), referenced from:
[error]     constructor at Test$Baz$1()(Test.scala:6)
[error]          method at Test$.main$$anonfun$1(Test.scala:9)
[error]          method at Test$$$Lambda$1.apply$mcVI$sp(Test.scala:9)
[error]          method at Test$.main(Test.scala:9)
[error]          method at Test.main(Test.scala:2)
[error] 
[error] Found unknown type java.sql.Types, referenced from:
[error]       type at Test$Baz$1(Test.scala:6)
[error]     method at Test$.main$$anonfun$1(Test.scala:9)
[error]     method at Test$$$Lambda$1.apply$mcVI$sp(Test.scala:9)
[error]     method at Test$.main(Test.scala:9)
[error]     method at Test.main(Test.scala:2)
[error] 
[error] Found unknown constructor java.sql.Time(long), referenced from:
[error]     constructor at Test$Foo$1(long)(Test.scala:4)
[error]          method at Test$.main$$anonfun$1(Test.scala:8)
[error]          method at Test$$$Lambda$1.apply$mcVI$sp(Test.scala:9)
[error]          method at Test$.main(Test.scala:9)
[error]          method at Test.main(Test.scala:2)
[error] 
[info] Total (1508 ms)
[error] Unreachable symbols found after classloading run. It can happen when using dependencies not cross-compiled for Scala Native or not yet ported JDK definitions.