fix(tree): add fallthrough switch markers

[aws-nslick]

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fix(tree): add fallthrough switch markers

Signed-off-by: Nicholas Sielicki nslick@amazon.com

[aws-nslick]

On the top PR, this was amended to use the following macro instead:

#if __has_attribute(__fallthrough__)
# define fallthrough                    __attribute__((__fallthrough__))
#else
# define fallthrough                    do {} while (0)  /* fallthrough */
#endif

Only GCC accepts the comments, clang requires the attribute. GCC also has the attribute. Prefer the attribute if possible, otherwise, use the comment.