duplicates macro arguments before expansion

As discussed with Jason, this is an important dimension of defenses that
we can build to ensure robustness of the macro engine.

This commit is important in the context of the upcoming patch to the
presentation compiler that will throw away expansions and keep original
macro applications (only when run in presentation compiler mode) so that
hyperlinking in macro arguments starts working in the IDE.

Duplication of macro arguments will make sure that macro arguments, which
are going to become exposed to the IDE, can’t become corrupted by possibly
misbehaving or misguided macros.
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)
Remove obsolete diagnostic error for SI-6231

The previous commit means we no longer have a test that
gets to this point.
Merge pull request #3257 from xeno-by/topic/duplicate-macro-args

(2.11.0-M8) duplicates macro arguments before expansion
Merge pull request #3234 from retronym/ticket/5508-3

 Fix crasher with private[this] in nested traits