[ASan] [Windows] Fix a new test in mingw mode
The new test case added in 781719778003ca298ca57e486ab629b7f384844b
didn't work in mingw mode; defines set through `/D` only works when
using the clang-cl compiler driver, not the regular clang compiler
driver.
Switch this to use `-D`, which works with both plain clang and
clang-cl, so we don't need to use a lit macro for expanding
the right option to use.
