Handle macros that have themselves as original tree (Fix #1237)

It has been reported in sbt/sbt#1237 that stack overflows may occur during the
extraction of used names (and later of dependencies between files). This
problem has been introduced by sbt/sbt#1163, which was about recording the
dependencies of macro arguments.

When a macro is expanded, the compiler attaches the tree before expansion to
the tree representing the expanded macro. As of Scala 2.11-RC3, some macros
have themselves attached as original tree, which caused the same macro to be
inspected over and over until a stack overflow.