SI-7689 Fix typing regression with default arguments

Regressed in e28c3edda4. That commit encoded upper/lower bounds
of Any/Nothing as EmptyTree, which were triggering the .isEmpty
check in Namers#TypeTreeSubstitutor and resulting in the default
getter having TypeTree() as the return type. This resulted in a
"missing parmameter type" error.

This commit tightens up that condition to only consider empty
TypeTrees (those wrapping null or NoType.)
MergeableEither extends AnyVal
Removing unused things from scalap.
fix Promise scaladoc
fix typo. s/Universes/Universe
SI-7695 Macro debug output on -explaintypes

It looks like checkConforms should wait for error before
using explainTypes, rather than turning on macro debug
trace if explaintypes is set.

In particular, macro debug output is Console.println
instead of reporter.echo or similar, so partest can't
test it.
Merge pull request #2779 from xuwei-k/Universe-typo

fix typo. s/Universes/Universe
Merge pull request #2778 from xuwei-k/fix-promise-scaladoc

fix Promise scaladoc
Merge pull request #2773 from folone/no-scalax

Cleanup of scalap.
Merge pull request #2772 from som-snytt/issue/6123-fix-test

SI-7695 Macro debug output on -explaintypes
Merge pull request #2766 from xuwei-k/request/mergeable-either-value-class

MergeableEither extends AnyVal
Merge pull request #2765 from retronym/ticket/7689

SI-7689 Fix typing regression with default arguments
Merge pull request #2757 from soc/SI-7681-dead-code-changes

SI-7681 Remove s.t.n.d.Changes (dead code)
Merge pull request #2758 from soc/SI-7681-dead-code-phases

SI-7681 Remove Phases, clean up TableDef
Merge pull request #2756 from soc/SI-7681-dead-code-settinggroup

SI-7681 Remove dead code in s.t.n.s.AbsScalaSettings
Merge pull request #2754 from soc/SI-7679-remove-standardscalasettings-make

SI-7679 Remove deprecated StandardScalaSettings#make
Merge pull request #2751 from soc/SI-7592-defaultmapmodel

SI-7592 Deprecate s.c.m.DefaultMapModel
Merge pull request #2733 from soc/SI-6811-text

SI-6811 Deprecate scala.text
SI-7624 Fix -feature warnings and build with -feature

I left one warning untouched, because instead of adding the import the
diagnostics should be fixed:

    scala/src/library/scala/collection/parallel/mutable/LazyCombiner.scala:33:
    warning: the existential type
        scala.collection.parallel.mutable.LazyCombiner[_$1,_$2,_$3] forSome {
          type _$1; type _$2; type _$3 <: scala.collection.generic.Growable[_$1] with scala.collection.generic.Sizing },
    which cannot be expressed by wildcards, should be enabled by making the implicit value scala.language.existentials visible.
    if (other.isInstanceOf[LazyCombiner[_, _, _]]) {
                       ^
SI-7624 Fix -Xlint warnings in AnyVal-related code