Fix the IntegersSubsetTest unit test when compiled with gcc-4.7. The issue here

is that the unit test doesn't have IntTy equal to APInt, instead it uses a class
derived from APInt.  When, as in these lines, an IntTy& reference is returned
but is assigned to an APInt&, the compiler destroys the temporary the IntTy& was
referring to, leaving the APInt& referring to garbage.  This causes the unittest
to fail systematically on my machine; it can also be caught by running the test
under valgrind.

llvm-svn: 167356