Improve pretty printer for comma-separated VarDecls and FieldDecls

[pgoodman]

int a __attribute__((annotate("a"))), b __attribute__((annotate("b")));

In the above, we'll print this roughly as:

int a __attribute__((annotate("a")));
int b __attribute__((annotate("b")))

This affects the algorithm in PrintedTokenRange::AlignTokens.