Add test and fix bug leading to infinite recursion.

Add test and fix bug leading to infinite recursion.

The test case here is simply "#define foo foo" and "#define bar foo"
and then attempting to expand "bar".

Previously, our termination condition for the recursion was overly
simple---just looking for the single identifier that began the
expansion. We now fix this to maintain a stack of identifiers and
terminate when any one of them occurs in the replacement list.