Improve ExplainState type handling in header files

Improve ExplainState type handling in header files

Now that we can have repeat typedefs with C11, we don't need to use
"struct ExplainState" anymore but can instead make a typedef where
necessary.  This doesn't change anything but makes it look nicer.

(There are more opportunities for similar changes, but this is broken
out because there was a separate discussion about it, and it's
somewhat bulky on its own.)

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/f36c0a45-98cd-40b2-a7cc-f2bf02b12890%40eisentraut.org#a12fb1a2c1089d6d03010f6268871b00
Discussion: https://www.postgresql.org/message-id/flat/10d32190-f31b-40a5-b177-11db55597355@eisentraut.org