[Bazel] Set the right default for LLVM_WINDOWS_PREFER_FORWARD_SLASH on Windows

This cmake configure option was added in
df0ba47, and was ported to
Bazel in 7d323dc.

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

Author: GMNGeoffrey

utils/bazel/llvm-project-overlay/llvm/config.bzl

@@ -36,7 +36,6 @@ posix_defines = [
     "HAVE_STRERROR_R=1",
     "HAVE_SYSEXITS_H=1",
     "HAVE_UNISTD_H=1",
-    "LLVM_WINDOWS_PREFER_FORWARD_SLASH=0",
 ]
 
 linux_defines = posix_defines + [
@@ -71,7 +70,6 @@ win32_defines = [
     # LLVM features
     r'LTDL_SHLIB_EXT=\".dll\"',
     r'LLVM_PLUGIN_EXT=\".dll\"',
-    "LLVM_WINDOWS_PREFER_FORWARD_SLASH=1",
 ]
 
 # TODO: We should switch to platforms-based config settings to make this easier

utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h

@@ -35,6 +35,10 @@
 /* Define to 1 to enable crash memory dumps, and to 0 otherwise. */
 #define LLVM_ENABLE_CRASH_DUMPS 0
 
+/* Define to 1 to prefer forward slashes on Windows, and to 0 prefer
+   backslashes. */
+#define LLVM_WINDOWS_PREFER_FORWARD_SLASH 0
+
 /* Define to 1 if you have the `backtrace' function. */
 /* HAVE_BACKTRACE defined in Bazel */