[tools] fix clang update script for clang download failure

If update script failed while downloading clang, this script left
inconsistency state between stamp and clang state (crbug.com/968765).

Let me remove directory having stamp so that next update
correctly downloads clang when previous download failed.

Bug: 968821
Change-Id: I095e4b037b9f513908551dfb1eee2496ace5936a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635087Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665119}

tools/clang/scripts/update.py

@@ -248,8 +248,8 @@ def UpdateClang():
   if ReadStampFile(STAMP_FILE) == expected_stamp:
     return 0
 
-  if os.path.exists(LLVM_BUILD_DIR):
-    RmTree(LLVM_BUILD_DIR)
+  if os.path.exists(os.path.dirname(LLVM_BUILD_DIR)):
+    RmTree(os.path.dirname(LLVM_BUILD_DIR))
 
   DownloadAndUnpackClangPackage(sys.platform, LLVM_BUILD_DIR)
   if 'win' in target_os: