clang_wrapper: More fixes to get V8 working.

[copybara-service]

clang_wrapper: More fixes to get V8 working.

• Automatically select indexer threads and merge queues for the indexer binary.
  Indexing V8 seems to be very slow.

• Ignore simdutf.cpp: This file seems seems to break the indexer with

Failed to run action on /src/v8/third_party/simdutf/simdutf.cpp

F0901 05:21:50.920894    6185 queue_state.cc:90] Check failed: (current & from) != 0 (0 vs. 0) Found kTaken while advancing kFinished -> kTaken
*** Check failure stack trace: ***
    @           0xe3bbc4  absl::lts_20250127::log_internal::LogMessage::SendToLog()
    @           0xe3b5f2  absl::lts_20250127::log_internal::LogMessage::Flush()
    @           0xe3c079  absl::lts_20250127::log_internal::LogMessageFatal::~LogMessageFatal()
    @           0xe2821a  oss_fuzz::indexer::QueueState::Advance()
    @           0xe263e5  oss_fuzz::indexer::SingleThreadMergeQueue::TakeIndex()
    @           0xe26b26  oss_fuzz::indexer::ParallelMergeQueue::TakeIndex()
    @           0xe26acf  oss_fuzz::indexer::ParallelMergeQueue::Cancel()
    @           0xc92211  main
    @          0x31e33a0  __libc_start_main

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