UBSan: Enable runtime library tests on Windows, and get most tests passing.

Specifically:

- Disable int128 tests on Windows, as MSVC cl.exe does not support
  int128, so we might not have been able to build the runtime
  with int128 support.

- XFAIL the vptr tests as we lack Microsoft ABI support.

- XFAIL enum.cpp as UBSan fails to add the correct instrumentation code
  for some reason.

- Modify certain tests that build executables multiple times to use
  unique names for each executable. This works around a race condition
  observed on Windows.

- Implement IsAccessibleMemoryRange for Windows to fix the last
  misaligned.cpp test.

- Introduce a substitution for testing crashes on Windows using
  KillTheDoctor.

Differential Revision: http://reviews.llvm.org/D10864

llvm-svn: 241303