[clang] [MinGW] Default to DWARF 4

Neither LLDB nor GDB seem to work with DWARF 5 debug information on
Windows right now.

This applies the same change as in
9c627286 (Default to DWARFv4 on Windows)
to the MinGW driver too.

Differential Revision: https://reviews.llvm.org/D119326

(cherry picked from commit 6cf64b2d)