Correct CFGBlock's front() and back() to return by const ref rather than value.

This ought to fix PR11926, a crash when when running Clang built with GCC 4.7

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155805 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed