[nomaster] doesn't touch NonemptyAttachments

NonemptyAttachments in macro API used to be Attachments$NonemptyAttachments.
However it's private, so noone outside scala.reflect.macros can access it,
making it ineligible for the incompatibility criterion.
[nomaster] removes Tree.canHaveAttrs

This forward compatibility fix is very easy. Have a new method in 2.10.1?
Don't expose it in the public API, and you won't have any problems.
[nomaster] revives BuildUtils.emptyValDef

That method was unnecessary in the first place, because we already had
emptyValDef in scala.reflect.api.Trees: https://github.com/scala/scala/blob/v2.10.0/src/reflect/scala/reflect/api/Trees.scala#L2367.
That's a rudiment from the times when we were unsure what to put into
scala.reflect.base and what goes into scala.reflect.api.

Unfortunately, it's not just a harmless extraneous method. Reifier v2.10.0
emits `u.build.emptyValDef` when it needs to reify `emptyValDef`. Therefore
we need to restore it to be forward compatible.

However we don't need to rollback the changes to the reifier v2.10.1,
which now simply calls `u.emptyValDef`, because, as mentioned above,
that `emptyValDef` was already there in v2.10.0.
[nomaster] Revert "s/SuperCallArgs/SuperArgs/"

This reverts commit 2621918870e531b7a9bcccdf14bddea9b89804b2.
[nomaster] Revert "more ListOfNil => Nil"

This reverts commit bb9adfbd76af0a0281912fcef4bcaca409a7c9a3.
[nomaster] Revert "DummyTree => CannotHaveAttrs"

This reverts commit 838cbe623c142b7005446793948097f679219fe3.

Conflicts:
	src/reflect/scala/reflect/api/Trees.scala
[nomaster] Revert "introduces global.pendingSuperCall"

This reverts commit 0ebf72b9498108e67c2133c6522c436af50a18e8.

Conflicts:
	src/compiler/scala/tools/nsc/typechecker/Typers.scala
	src/reflect/scala/reflect/internal/Trees.scala
[nomaster] Revert "refactors handling of parent types"

This reverts commit 40063b0009d55ed527bf1625d99a168a8faa4124.

Conflicts:
	src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
	src/compiler/scala/tools/nsc/typechecker/Typers.scala
[nomaster] verifies compat with 2.10.0

Makes sure that the compiler understands the 2.10.0 format of super
constructor calls after the recent revert from the typemacro-compatible
format.
Merge pull request #2068 from scalamacros/ticket/7064

[nomaster] SI-7064 Reflection: forward compat for 2.10.1