Add Bazel support for LLVM_WINDOWS_PREFER_FORWARD_SLASH

This was added in df0ba47c36

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

@@ -36,6 +36,7 @@ posix_defines = [
     "HAVE_STRERROR_R=1",
     "HAVE_SYSEXITS_H=1",
     "HAVE_UNISTD_H=1",
+    "LLVM_WINDOWS_PREFER_FORWARD_SLASH=0",
 ]
 
 linux_defines = posix_defines + [
@@ -70,6 +71,7 @@ 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_configs/config.h.cmake

@@ -19,6 +19,10 @@
 /* Define to 1 to enable crash memory dumps, and to 0 otherwise. */
 #cmakedefine01 LLVM_ENABLE_CRASH_DUMPS
 
+/* Define to 1 to prefer forward slashes on Windows, and to 0 prefer
+   backslashes. */
+#cmakedefine01 LLVM_WINDOWS_PREFER_FORWARD_SLASH
+
 /* Define to 1 if you have the `backtrace' function. */
 #cmakedefine HAVE_BACKTRACE ${HAVE_BACKTRACE}