SI-8129 Crack the case of the curiously incoherent Context

 - Typer is created with Context.
 - Typer creates an Inferencer with said Context.
 - Typer mutates Typer#context after each import statement
 - Typer mutates its current Context (e.g to disable implicits.)
 - Typer asks a question of Inferencer
 - Inferencer, looking at the old context, thinks that implicits
   are allowed
 - Inferencer saves implicit ambiguities into the wrong Context.

Because of this bug, overload resolution in blocks or template
bodies for applications that follow an import have been
considering implicit coercions in the first try at static overload
resolution, and, in the rare case that it encounters an ambigous
implicit in the process, leaking an unpositioned ambiguout error.

This commit ensures coherency between `typer.context` and
`typer.infer.context` by making the latter delegate to the former.

src/compiler/scala/tools/nsc/typechecker/Infer.scala

@@ -174,7 +174,8 @@ trait Infer extends Checkable {
   private lazy val stdErrorValue = stdErrorClass.newErrorValue(nme.ERROR)
 
   /** The context-dependent inferencer part */
-  class Inferencer(context: Context) extends InferencerContextErrors with InferCheckable {
+  abstract class Inferencer extends InferencerContextErrors with InferCheckable {
+    def context: Context
     import InferErrorGen._
 
     /* -- Error Messages --------------------------------------------------- */

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -109,7 +109,8 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
 
     private val transformed: mutable.Map[Tree, Tree] = unit.transformed
 
-    val infer = new Inferencer(context0) {
+    val infer = new Inferencer {
+      def context = Typer.this.context
       // See SI-3281 re undoLog
       override def isCoercible(tp: Type, pt: Type) = undoLog undo viewExists(tp, pt)
     }