Faster Clang plugins

 Hello,

 we currently have about 75 checks in our Clang plugin, and as I said in my 
talk at the conference, this adds some overhead during compilation (+ ~25%). 
During a recent Collabora Hackweek I had a look at trying to merge all their 
AST passes into one, which should noticeably reduce the time. I've just 
pushed a commit with 25 plugins converted, which drops the overhead to ~17%. 
So, 2 things:

- If something breaks there, such as getting needless rebuilds, please 
complain :).

- The conversion of the rest of the plugins still should be done, getting the 
overhead to estimated ~5%, if all can be converted. I'll try to do these 
gradually as time permits, which may take time, so if somebody would like to 
help, please feel free to. There's description at the top of 
compilerplugins/clang/sharedvisitor/generator.cxx and changes done in 
adb08e892b37ea9 should also help (and if necessary, I'll help too).

-- 
 Luboš Luňák
 l.lunak@collabora.com