Reverted "block_x" to "block" in the header and merged.

Looks good to me. I would not have changed "block" to "block_x" in the header
file, but feel free to include that change if you consider it an improvement.

I merged the current master into the issue branch.

Change title to reflect that we don't want to enforce using the same variable 
names in function declarations and definitions.

I agree that a clean interface is more desirable than consistent names. I don't 
think (anymore) that we should enforce that the same names are used in the 
declaration and definition and therefore disabled the clang-tidy check again 
and reverted most of the changes.

I think the current patch makes some names clearer and adds a bit of 
consistency, but I'd also be fine with not merging it.

I've left a bunch of comments. (See also
http://www.fast-downward.org/ForDevelopers/CodingConventions under
"Anti-idioms". You added the relevant line in revision 19 of that page.)

I don't mind a bit of consistency, but I think the added value here is
questionable because many of the changes go in a bad direction, and I think
there are better ways to use of our resources. But if nobody is unhappy with it,
I don't mind the change if you can address the comments on bitbucket.

I made a pull request at https://bitbucket.org/jendrikseipp/downward/pull-requests/86 .