Clang update script: bump gnuwin version to include cut.exe

LLVM r365162 added a lit tests that uses cut. The new package has
cut.exe from UnxUtils.zip.

R=hans@chromium.org, pcc@chromium.org, thakis@chromium.org

Bug: 984065
Change-Id: I35c42c33cc3c621ad5881caffc50d80f2cf11ed9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707474
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Auto-Submit: Reid Kleckner <rnk@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678487}

tools/clang/scripts/build.py

@@ -212,7 +212,7 @@ def AddGnuWinToPath():
     return
 
   gnuwin_dir = os.path.join(LLVM_BUILD_TOOLS_DIR, 'gnuwin')
-  GNUWIN_VERSION = '11'
+  GNUWIN_VERSION = '12'
   GNUWIN_STAMP = os.path.join(gnuwin_dir, 'stamp')
   if ReadStampFile(GNUWIN_STAMP) == GNUWIN_VERSION:
     print('GNU Win tools already up to date.')