Clang build.py: pass -DCOMPILER_RT_USE_LIBCXX=NO

When we switched to building clang from the monorepo, libfuzzer started pulling
in libcxx and libcxxabi. It makes the binary larger, and more importantly, it
doesn't build successfully on the ToT bots.

TBR=thakis

Bug: 897796
Change-Id: Ia54595c8295a3eb1874c2deb3c190b4ac860f4ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1624186Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662111}

tools/clang/scripts/build.py

@@ -385,6 +385,8 @@ def main():
                      # TODO(crbug.com/929645): Use newer toolchain to host.
                      '-DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN=ON',
                      '-DBUG_REPORT_URL=' + BUG_REPORT_URL,
+                     # See PR41956: Don't link libcxx into libfuzzer.
+                     '-DCOMPILER_RT_USE_LIBCXX=NO',
                      ]
 
   if sys.platform == 'win32':