Unfortunately, this requires partially reverting
https://boringssl-review.googlesource.com/31324. This is a mess.

While clang proper includes a fuzzer driver, Chromium doesn't use it.
Chromium builds exclusively with fuzzer-no-link and links to its own
copy of the fuzzer runtime[1]. As of [2], Chromium's clang (which we use
on bots) no longer includes the driver, so we must mimic them.

However, Chromium's setup is somewhat questionable because
fuzzer-no-link pulls in libclang_rt.fuzzer_no_main which still includes
most of libclang_rt.fuzzer, just not the one main function[3]. It
appears Chromium is actually linking two copies of
libclang_rt.fuzzer_no_main. Hopefully this mostly works out as Chromium's
clang and libFuzzer should be relatively aligned, but it's not a good
assumption for our build, which can take other Clangs too.

Thus, if you pass -DFUZZ=1 as-is, we will assume you are using a
"normal" Clang with all its relevant runtimes intact. If, however, you
are using Chromium clang, you must drop the matching libFuzzer where the
bots expected it and build with -DLIBFUZZER_FROM_DEPS=1.

This involves no changes to the bots because we never actually unwound
all the LIBFUZZER_FROM_DEPS bits before.

[1] https://cs.chromium.org/chromium/src/testing/libfuzzer/BUILD.gn?rcl=d21c49585f262e851e2984f96f52905782706325&l=14
[2] c79bf2ea4c
[3] 8ebc3668b0/lib/fuzzer/CMakeLists.txt (L93-L107)
    8ebc3668b0/lib/fuzzer/FuzzerMain.cpp

Change-Id: I946b3c821c3d7e6def7e07f1381f58241611ba3d
Reviewed-on: https://boringssl-review.googlesource.com/c/34184
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>