[Bazel] Set the right default for LLVM_WINDOWS_PREFER_FORWARD_SLASH on Windows

This cmake configure option was added in
df0ba47c, and was ported to
Bazel in 7d323dc7.

However, the setting chosen in Bazel seems accidental, not necessarily
intentional.

LLVM_WINDOWS_PREFER_FORWARD_SLASH has no effect on Unix, and on
Windows, setting it to 0 is the default, which gets the same behaviour
as before. Setting it to 1 enables new experimental behaviours
(which is enabled by default on MinGW targets only).

As I don't see any explicit intent to opt in to the new experimental
behaviour, I believe the current configuration in bazel was a
mistake.

Differential Revision: https://reviews.llvm.org/D114065