Fix LLVM tool --version build mode printing for MSVC

LLVM tools such as llc print "DEBUG build" or "Optimized build" when
passed --version. Before this change, this was implemented by checking
for the __OPTIMIZE__ GCC macro. MSVC does not define this macro. For
MSVC, control this behavior with _DEBUG instead. It doesn't have
precisely the same meaning, but in most configurations, it will do the
right thing.

Fixes PR17752

Reviewed by: MaskRay

Differential Revision: https://reviews.llvm.org/D71817