Use stack-allocated StringInfoData

Hi all,

While working on other things I noted that we have a lot of cases where 
a StringInfo instance is allocated dynamically even when it is either 
thrown away or destroyed at the end, which seems unnecessary, that is, 
instead of using:

        StringInfo info = makeStringInfo();
        ...
        appendStringInfo(info, ...);
        ...
        return info->data;

We can use

        StringInfoData info;
        initStringInfo(&info);
        ...
        appendStringInfo(&info, ...);
        ...
        return info.data;

It was corrected in an earlier commit, but that seems to have been 
removed so we still have a lot of these cases.

I created a semantic patch to capture most of these cases, which is 
present in [1], but this is a slightly modified version that might be 
interesting to include regardless of other changes. The patch is applied 
and one case that couldn't be matched is manually fixed.

[1]: 
https://www.postgresql.org/message-id/8895cba9-48cf-40fe-9c47-9b43ec6b2ab3%40gmail.com

Best wishes,
Mats Kindahl