Windows: Fix clang/scripts/update.py failure

Fix clang/scripts/update.py failure with DEPOT_TOOLS_WIN_TOOLCHAIN=0
and non-standard VS install location.

If GYP_MSVS_OVERRIDE_PATH is set that value should be used instead of
calling vs_toolchain.DetectVisualStudioPath().

R=brucedawson@chromium.org

Bug: 863130
Change-Id: Id72b1a22f74c5fa9174a76d611fef9ca69bb8319
Reviewed-on: https://chromium-review.googlesource.com/1135761Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Marshall Greenblatt <marshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579056}

tools/clang/scripts/update.py

@@ -392,7 +392,10 @@ def GetWinSDKDir():
   if bool(int(os.environ.get('DEPOT_TOOLS_WIN_TOOLCHAIN', '1'))):
     dia_path = os.path.join(win_sdk_dir, '..', 'DIA SDK', 'bin', 'amd64')
   else:
-    vs_path = vs_toolchain.DetectVisualStudioPath()
+    if 'GYP_MSVS_OVERRIDE_PATH' not in os.environ:
+      vs_path = vs_toolchain.DetectVisualStudioPath()
+    else:
+      vs_path = os.environ['GYP_MSVS_OVERRIDE_PATH']
     dia_path = os.path.join(vs_path, 'DIA SDK', 'bin', 'amd64')
 
   dia_dll = os.path.join(dia_path, DIA_DLL[msvs_version])