[analyzer] Enable usage of temporaries in InitListExprs

Summary:
ExprEngine had code which specificaly disabled using CXXTempObjectRegions in
InitListExprs. This was a hack put in r168757 to silence a false positive.

The underlying problem seems to have been fixed in the mean time, as removing
this code doesn't seem to break anything. Therefore I propose to remove it and
solve PR16629 in the process.

Reviewers: jordan_rose

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1325