clang/test/Driver/fsanitize.c: Fix -fsanitize=vptr using default target

The default implementation of getSupportedSanitizers isn't able to turn
on the vptr sanitizer, and thus, any platform that runs this test will
fail with the error:

    clang: error: unsupported option '-fsanitize=vptr' for target '<target>'

Patch by James Nagurne!

llvm-svn: 365981