SI-7854, SI-6768 better parsing/positioning in parser

The parser hole I found while working on the generated positions
serves as the umbrella for a host of improvements. Upgraded
positions assigned during some specific challenging situations mostly
involving the creation of synthetic trees, e.g. for comprehensions
and closures. While doing so improved some error messages.

Eliminated some of the most glaring duplication in the parser.
It's written like there is some payoff associated with being
spectacularly imperative. Not so far.

Author: paulp

test/files/continuations-run/t5472.check

@@ -5,7 +5,7 @@ t5472.scala:12: warning: expression case4(){
     case5()
 } is cps-transformed unexpectedly
           (location, accessors) <- new ContinuationizedParallelIterable(map)
-                                   ^
+                                       ^
 t5472.scala:12: warning: expression matchEnd3(x: Int){
   x
 } is cps-transformed unexpectedly