[llvm-profdata] Fix mixed-sign comparison warnings

This patch fixes -Wsign-compare warnings from:

  llvm/unittests/tools/llvm-profdata/MD5CollisionTest.cpp:117:3: note:
  in instantiation of function template specialization
  'testing::internal::EqHelper::Compare<unsigned long, int, nullptr>'
  requested here

  llvm/unittests/tools/llvm-profdata/MD5CollisionTest.cpp:134:3: note:
  in instantiation of function template specialization
  'testing::internal::EqHelper::Compare<unsigned int, int, nullptr>'
  requested here

  llvm/unittests/tools/llvm-profdata/MD5CollisionTest.cpp:144:3: note:
  in instantiation of function template specialization
  'testing::internal::EqHelper::Compare<unsigned int, int, nullptr>'
  requested here

  llvm/unittests/tools/llvm-profdata/MD5CollisionTest.cpp:160:3: note:
  in instantiation of function template specialization
  'testing::internal::EqHelper::Compare<unsigned int, int, nullptr>'
  requested here