Bringing a bit of order to symbol substitution.

Painstakingly winnowed out the most frequently duplicated code sequences
related to symbol cloning and substitution. Created canonical methods to
perform these actions and documented them. Key methods include:

  def createFromClonedSymbols[T](syms: List[Symbol], tpe: Type)(creator: (List[Symbol], Type) => T): T
  def deriveSymbols(syms: List[Symbol], symFn: Symbol => Symbol): List[Symbol]
  def deriveType(syms: List[Symbol], symFn: Symbol => Symbol)(tpe: Type): Type

Many example usages enclosed with commit.

I did lots of timing tests, I find no material difference before and
after. Actually I won by four seconds in this incarnation:

  Before - Total time: 7 minutes 55 seconds
   After - Total time: 7 minutes 51 seconds

Review by moors.

1 files changed, 2 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Namers.scala b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
index 9e0f70a32c..fe474e5d3d 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Namers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
@@ -130,9 +130,8 @@ trait Namers extends MethodSynthesis {
     )
 
     def setPrivateWithin[Sym <: Symbol](tree: Tree, sym: Sym, mods: Modifiers): Sym = {
-      if (!sym.isPrivateLocal && mods.hasAccessBoundary)
-        sym.privateWithin = typer.qualifyingClass(tree, mods.privateWithin, true)
-      sym
+      if (sym.isPrivateLocal || !mods.hasAccessBoundary) sym
+      else sym setPrivateWithin typer.qualifyingClass(tree, mods.privateWithin, true)
     }
     def setPrivateWithin(tree: MemberDef, sym: Symbol): Symbol =
       setPrivateWithin(tree, sym, tree.mods)