The issue with just passing `-gcodeview` is that it's not part of
the `OPT_g_Group` in `clang/Driver/Options.td`, so it doesn't trigger
debug info to be generated.

Passing only `-g` is sufficient on MSVC, as there is logic in `Clang.cpp`
which enables codeview if that is the default for the toolchain. Since
the default for -gnu is not codeview, we do pass `-gcodeview` in that case.