[android wrapper] Write warning report to stdout on Android

Write the warning report to stdout (surrounded by a tag
<LLVM_NEXT_ERROR_REPORT>).  Currently OUT_DIR only gets passed to local
compiles, and so remote compiles can't do the double build.  Even if
they write to /tmp, RBE or Goma won't know to copy them as part of the
output.

Packaging the warnings reports is a problem as well since the additional
build step needs extra surgery in soong (it's not possible to add a
dangling, optional build step in Ninja).

Writing the reports to stdout conveniently solves both these issues.  We
can just scrape the reports with a script.

Originall reviewed at
https://android-review.googlesource.com/c/platform/external/toolchain-utils/+/1328433/2.

BUG=None
TEST=build master with r391452 and check progress past new errors.

Change-Id: I5f1f71faba002836067a82f6aa4b26d5ba8b7b99
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2239371
Commit-Queue: Pirama Arumuga Nainar <pirama@google.com>
Tested-by: Pirama Arumuga Nainar <pirama@google.com>
Reviewed-by: George Burgess <gbiv@chromium.org>
3 files changed