Move token start position appropriately when stripping __identifer__ prefix

Fixes 1043622#commitcomment-133811801

Author: hsutter

source/lex.h

@@ -307,6 +307,7 @@ class token
             )
         {
             sv.remove_prefix(prefix.size());
+            pos.colno += prefix.size();
         }
     }
 

source/parse.h

@@ -8264,7 +8264,6 @@ class parser
 
         else
         {
-
             //  Next is optionally a requires clause (if not using the "-> expr;" syntax)
             if (curr() == "requires")
             {