[clang] Don't default to a specifically shared libunwind on mingw with a g++ driver

For MinGW targets, we distinguish between an explicitly shared unwinder
library (requested via -shared-libgcc), an explicitly static one
(requested via -static-libgcc or -static) and the default case (which
just passes -lunwind to the linker, which will pick either shared or
static depending on what's available, with the normal linker logic).

This makes the implicit default case (as added in D79995) actually work as
it was intended, when using the g++ driver (which is the main usecase for
libunwind as far as I know).

Differential Revision: https://reviews.llvm.org/D98023