Add pending scripted test for Ant-style compilation.

Add pending test for Ant-style incremental compilation. In that mode
incremental compiler will recompile only the source files that were changed
by the user and won't try to invalidate any dependencies.

Once Ant-style incremental compilation is implemented this test should be
passing.
Add `antStyle` to IncOptions.

Add an option that enables (to be implemented) Ant-style mode of
incremental compilation.

This option is unsupported and may go away at any point in the future.

NOTE: Either `antStyle` or `nameHashing` mode can be enabled. This is
being enforced with runtime assertion.
Add Ant-style incremental compilation mode.

This commit implements an Ant-style incremental compilation mode. This mode
emulates what Ant's scalac command does. It recompiles just changed source
files and does not perform any invalidation of dependencies.
This is a very naive mode of incremental compilation that very often leads
to broken binaries.

The Ant-style mode is being introduced because Scala team needs it for
migration of Scala compiler to sbt. The name hashing algorithm doesn't
work well with Scala compiler sources due to deep inheritance chains.
There's a plan to refactor compiler's code to use more composition instead
of inheritance.

Once Scala compiler sources are refactored to work well with name hashing
algorithm, Ant-style mode will be deleted immediately.
Merge pull request #1317 from gkossakowski/ant-style-inc

Add Ant-style incremental compilation mode