[Support] Allow configuring the preferred type of slashes on Windows

Default to preferring forward slashes when built for MinGW, as
many usecases, when e.g. Clang is used as a drop-in replacement
for GCC, requires the compiler to output paths with forward slashes.

Not all tests pass yet, if configuring to prefer forward slashes though.

Differential Revision: https://reviews.llvm.org/D112787