Mark gloabl entry point @Unshared

The resident compiler is not supposed to be called from multiple
threads; mark as unshared to avoid spurious re-entrancy errors.

Author: odersky

src/dotty/tools/dotc/Resident.scala

@@ -23,7 +23,7 @@ import scala.annotation.tailrec
  */
 object Resident extends Driver {
 
-  object residentCompiler extends Compiler
+  @unshared object residentCompiler extends Compiler
 
   override def newCompiler(): Compiler = ???