Silence gcc warning in testcase [NFC]

Without the fix gcc (7.4.0) complains with

../unittests/ADT/APIntTest.cpp: In member function 'virtual void {anonymous}::APIntTest_MultiplicativeInverseExaustive_Test::TestBody()':
../unittests/ADT/APIntTest.cpp:2510:36: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
     for (unsigned Value = 0; Value < (1 << BitWidth); ++Value) {
                              ~~~~~~^~~~~~~~~~~~~~~~~

llvm-svn: 364624