Switch users of the 'for (StmtRange range = stmt->children(); range; ++range)‘ pattern to range for loops.

The pattern was born out of the lack of range-based for loops in C++98
and is somewhat obscure. No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241300 91177308-0d34-0410-b5e6-96231b3b80d8