[clang] [clang-format] Stop crashing when the input contains `??/\n` (PR #147156)

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sstwcw wrote:

I am not sure we should support the trigraph.  I don't know anyone who uses it.  The C2y draft does not mention it.  Not even in the change log that it got removed.  To stop the program from crashing, should I remove all the code for handling it instead?

https://github.com/llvm/llvm-project/pull/147156