SI-5508 Fix crasher with private[this] in nested traits

Currently, accessors for private local trait fields are added
very late in the game when the `Mixin` tree transformer treats the
trait. By contrast, fields with weaker access have accessors created
eagerly in `Namers`.

    // Mixin#addLateInterfaceMembers
    val getter = member.getter(clazz)
    if (getter == NoSymbol) addMember(clazz, newGetter(member))

`addMember` mutates the type of the interface to add the getter.
(This seems like a pretty poor design: usually if a phase changes
types, it should do in an `InfoTransformer`.)

However, if an inner class or anonymous function of the trait
has been flattened to a spot where it precedes the trait in the
enclosing packages info, this code hasn't had a chance to run,
and the lookup of the getter crashes as mixins `postTransform`
runs over a selection of the not-yet-materialized getter.

    // Mixin#postTransform
	case Select(qual, name) if sym.owner.isImplClass && !isStaticOnly(sym) =>
	  val iface  = toInterface(sym.owner.tpe).typeSymbol
	  val ifaceGetter = sym getter iface

This commit ensures that `Flatten` lifts inner classes to
a position *after* the enclosing class in the stats of the
enclosing package.

Bonus fix: SI-7012 (the followup ticket to SI-6231 / SI-2897)

1 files changed, 4 insertions, 0 deletions

diff --git a/test/files/pos/t5508-min-okay2.scala b/test/files/pos/t5508-min-okay2.scala
new file mode 100644
index 0000000000..935f28609c
--- /dev/null
+++ b/test/files/pos/t5508-min-okay2.scala
@@ -0,0 +1,4 @@
+trait TopTrait { // must be nested and a trait
+  private[this] val _st : Int = 0 // crashes if TopTrait is not top level
+  val escape = { () => _st }
+}