Set currentRun and currentUnit in Eval

First of all, we revert changes to Eval made in
a9cdd96152e08a1fb21e815eabe8fd1d27ef9b97. That was a work-around
for problem with broken positions set on some trees. The rest of
the commit describes a fix.

Scala compiler's Global has `currentRun` method which is supposed to
return an instance of Run currently in use. Sbt's Eval which wraps
Global would create an instance of Run but not register it in Global.
Similarly, Eval would create an instance of `CompilationUnit` but not
set it in Run.

This would result in some silent failures like assigning broken positions
to parsed trees even if parser *has* received an instance of compilation
unit. See https://issues.scala-lang.org/browse/SI-8794 for details.

We fix the issue by subclassing Global and making it possible to set
current Run instance externally. Then we use this capability to set a run
instance right before we call compiler phases. We also make sure that
current run has current compilation assigned to it.

Fixes #1181
Fixes #1501
Fixes #1523