Clang update script: fix Android compiler-rt builds after r346167

NDK r16 "helpfully" installs libc++ as libstdc++ to make the NDK's Clang use
that by default. However, we use a more recent Clang, and after r346167 that
starts looking for libc++, which then can't be found due to the renaming.

This hacks around the problem by renaming it back. Well, it makes a copy
actually, to support Clangs both before and after. We should remove all this in
the future when we've moved to a newer NDK.

Bug: 902270
Change-Id: Ib69be5faeb0e0b0c8b5b2588fdb66610e30de687
Reviewed-on: https://chromium-review.googlesource.com/c/1319712
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Reviewed-by: Reid Kleckner <rnk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605772}

tools/clang/scripts/update.py

@@ -781,6 +781,14 @@ def UpdateClang(args):
               'i686': 'x86',
           }[target_arch]])
 
+      # NDK r16 "helpfully" installs libc++ as libstdc++ "so the compiler will
+      # pick it up by default". Only these days, the compiler tries to find
+      # libc++ instead. See https://crbug.com/902270.
+      shutil.copy(os.path.join(toolchain_dir, 'sysroot/usr/lib/libstdc++.a'),
+                  os.path.join(toolchain_dir, 'sysroot/usr/lib/libc++.a'))
+      shutil.copy(os.path.join(toolchain_dir, 'sysroot/usr/lib/libstdc++.so'),
+                  os.path.join(toolchain_dir, 'sysroot/usr/lib/libc++.so'))
+
       # Build compiler-rt runtimes needed for Android in a separate build tree.
       build_dir = os.path.join(LLVM_BUILD_DIR, 'android-' + target_arch)
       if not os.path.exists(build_dir):