Fix location of Clang stamp file.

It should be inside the llvm-build directory, otherwise it's not in
.gitignore and things get confused.

Change-Id: I5be31e0b0db69fff9935cbf6dbd9c612fd5a4769
Reviewed-on: https://boringssl-review.googlesource.com/21805
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

util/bot/update_clang.py

diff --git a/util/bot/update_clang.py b/util/bot/update_clang.py
index 9595c0a..70abf41 100644
--- a/util/bot/update_clang.py
+++ b/util/bot/update_clang.py
@@ -27,7 +27,7 @@
 # Path constants. (All of these should be absolute paths.)
 THIS_DIR = os.path.abspath(os.path.dirname(__file__))
 LLVM_BUILD_DIR = os.path.join(THIS_DIR, 'llvm-build')
-STAMP_FILE = os.path.join(THIS_DIR, 'cr_build_revision')
+STAMP_FILE = os.path.join(LLVM_BUILD_DIR, 'cr_build_revision')
 
 # URL for pre-built binaries.
 CDS_URL = os.environ.get('CDS_CLANG_BUCKET_OVERRIDE',