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

[Owen Pan via cfe-commits]

owenca wrote:

> I added code to count the number of characters in the escape sequence probably just because the block of code used to have a comment saying someone should add the feature.

> 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?

+1.

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