Updated Symbol to record classfile origin.

This change should be transparent to anything using sourceFile,
unless it was drinking from the inheritance well too deeply. Rather
than squander the already allocated field for every ClassSymbol not
being compiled from source, I'm now populating it with the file
representing the class. This will make a broad range of things easier,
like debugging, issuing useful error messages, symbol invalidation,
signature verification, you name it.

  def sourceFile      - still returns only source code files
  def binaryFile      - returns only class files
  def associatedFile  - returns whatever is there, if anything

Performance: I may be mistaken, but I believe this is a zero-impact
change.  No new fields are allocated; fields which were null now hold
a useful reference.  The reference is to a file instance which was
already being allocated and already long-lived.

Compare error messages:

  // Version 1
  % scalac a.scala
  error: type _$1 is defined twice

  // Version 2
  % scalac a.scala
  error: type _$1 is defined twice
    conflicting symbols both originated in file './foo/package.class'
    Note: this may be due to a bug in the compiler involving wildcards in package objects
  one error found

Bonus for people who read commit logs.  Try this in the repl
after starting power mode.

ListClass.info.members groupBy (_.associatedFile) foreach {
  case (k, vs) => println("%s\n  %s\n".format(k, vs map (_.defString) mkString "\n  "))
}

1 files changed, 2 insertions, 1 deletions

diff --git a/test/files/neg/t591.check b/test/files/neg/t591.check
index 5cdeebf079..d33f6d7a2f 100644
--- a/test/files/neg/t591.check
+++ b/test/files/neg/t591.check
@@ -1,4 +1,5 @@
-t591.scala:38: error: method input_= is defined twice in t591.scala
+t591.scala:38: error: method input_= is defined twice
+  conflicting symbols both originated in file 't591.scala'
     def input_=(in : Input) = {}
         ^
 one error found