[analyzer] Fix infinite recursion in printing macros

#define f(y) x
#define x f(x)
int main() { x; }

This example results a compilation error since "x" in the first line was not
defined earlier. However, the macro expression printer goes to an infinite
recursion on this example.

Patch by Tibor Brunner!

Differential Revision: https://reviews.llvm.org/D57892

llvm-svn: 354806