diff --git a/utils/bazel/llvm-project-overlay/llvm/config.bzl b/utils/bazel/llvm-project-overlay/llvm/config.bzl
--- a/utils/bazel/llvm-project-overlay/llvm/config.bzl
+++ b/utils/bazel/llvm-project-overlay/llvm/config.bzl
@@ -36,7 +36,6 @@
     "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 @@
     # 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
diff --git a/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h b/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h
--- a/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h
+++ b/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 */