Fix N^2 spot in erasure.

An expression containing nested casts would type the same tree 2^N
times where N is the number of nested casts. This was particularly
visible in Vector.scala where one can find a six-cast expression.
It's currently line "why was six afraid of seven", aka line 789.

  display5((index >> 25) & 31)
    .asInstanceOf[Array[AnyRef]]((index >> 20) & 31)
    .asInstanceOf[Array[AnyRef]]((index >> 15) & 31)
    .asInstanceOf[Array[AnyRef]]((index >> 10) & 31)
    .asInstanceOf[Array[AnyRef]]((index >> 5) & 31)
    .asInstanceOf[Array[AnyRef]](index & 31)
    .asInstanceOf[T]

Compiling Vector.scala, before/after.

  < #unique types                 : 10805
  > #unique types                 : 9722
  < #created tree nodes           : 26716
  > #created tree nodes           : 25647

I found a similar bug about a year ago in 39f01d4f48. It's
interesting to consider how little defense we have against bugs
of this nature - visually non-obvious differences in tree
traversal can have spectacular impact on complexity.
Par-Test split checks work again

Check files split into conditional blocks with partest flags
have been broken since the new diff regime.

For some reason, no one noticed.

The clever scheme to "filter the diff" instead of just filtering
the check file is abandoned as futile and unnecessary.

Fix java6 checkfile for ifdiff fix.
SI-7729 Does Par-Test work?  Absolutely!

Testicolor just passed file arguments through; this commit does
either toAbsolute or getAbsoluteFile (depending on the File API
impedance mismatch) at ConsoleRunner (command line) and Runner
(ant supplies absolute paths, but in case sbt runner doesn't,
and who knows). It also sprinkles toAbsolute internally, where
it may be redundant. Do I mean redundant or redolent?

I noticed that my branch from last year relativized to test root
always; so this is one of those choices one commits to.

The point of breakage in this case was, if a path is not absolute,
use test root / path, which only works (of course) if the relative
path is relative to test root, which is no longer the case.

Because all paths are now calculated with respect to the test file
path, the "out" dir would change its relative path depending on
the invocation.
Log file is zapped before test run

Not all test configurations handle an existing log file.

In particular, kicking off javac will append to the log file
and always fail the test.

Don't even ask how I know that.
DefDef.name is now TermName again

Now when there's no hope left for type macros, it's reasonable to provide
a more specific type for DefDef.name.
SI-7690 ghost error message fails compile

I won't even try to explain the error reporting code, because
it would have no basis in reality. However this change appears
to fix the symptom reported.
Merge pull request #2797 from paulp/pr/erasure-n-squared

Fix N^2 spot in erasure.
Merge pull request #2812 from xeno-by/topic/defdef-name-now-termname

DefDef.name is now TermName again
Merge pull request #2764 from folone/master

Making map2Conserve occupy constant stack space in spirit of SI-2411.
Merge pull request #2807 from som-snytt/issue/partest-dir

SI-7729 Does Par-Test work?  Absolutely!
Merge pull request #2804 from adriaanm/rebase-2763

SI-7690 ghost error message fails compile [Rebase of #2763]