Use fallthrough attribute instead of comment as the comment no longer works in Clang 10.

[BoboFraggins]

Clang 10 stopped supporting fallthrough comments in favor of the statement attribute 'fallthrough'.

For reference:

https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html

And, where the code maintainer of Clang explains the issue:

https://bugs.llvm.org/show_bug.cgi?id=43465#c37

He suggests either updating to the new style or turning off the warning. I assume you would prefer to not turn off the warning, although I don't know what version of gcc is used.

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