Recursive calls to ValueObject::GetSummaryAsCString() are causing crashes.

The previous approach to controlling the recursion was doing it from
outside the function which is not reliable. Now it is being done inside
the function. This might not solve all of the crashes that we were seeing
since there are other functions that clear the bit that indicates that
the summary is in the process of being generated, but it might solve some.

llvm-svn: 147741