Fix MSVC build

https://chromium-review.googlesource.com/c/chromium/src/+/6311850
removed the only usage _clang_lib_dir had on MSVC.
It is only used "if (use_lld)", so wrap the declaration with it as well.

Bug: 381477464, 400387818
Change-Id: Ie9fa6a544f09ef9bde8deee11da92abe42c4d3c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6316789
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1427097}

build/toolchain/win/toolchain.gni

@@ -776,9 +776,11 @@ template("win_toolchains") {
     clang_toolchain_prefix = "win_clang_"
   }
 
-  _clang_lib_dir =
-      rebase_path("$clang_base_path/lib/clang/$clang_version/lib/windows",
-                  root_build_dir)
+  if (use_lld) {
+    _clang_lib_dir =
+        rebase_path("$clang_base_path/lib/clang/$clang_version/lib/windows",
+                    root_build_dir)
+  }
 
   msvc_toolchain(clang_toolchain_prefix + target_name) {
     environment = "environment." + toolchain_arch