Fixed the C modules test case on Darwin by streamlining its code.

We don't need to do the fancy dance with checking whether the iterator
represents a #define -- in fact, that's the wrong thing to do.  The thing to do
is check whether the highest-priority module that did something to the module
#defined or #undefd it.  If it #defined it, then the MacroInfo* will be non-NULL
and we're good to go.

llvm-svn: 241651