Set AlignTrailingComments in the clang-format config

[arichardson]

Summary

This seems to be fairly common in existing code and often looks better when
adding trailing comments to e.g. enumerators or array initializers.
See D26340 for more context.

[arichardson]

correct diff

[emaste]

I'm curious how well this works in practice (i.e., it's often difficult to simultaneously address aligned comments and line length limits) but this is probably the right choice.