DebugInfo: Fix PR19864 better - attribute the jump at the end of a range-for loop, to the start of the loop.

This avoids debuggers stepping to strange places (like the last
statement in the loop body, or the first statement in the if).

This is not the whole answer, though - similar bugs no doubt exist in
other loops (patches to follow) and attributing exception handling code
to the correct line is also tricky (based on the previous fix to
PR19864, exception handling is still erroneously attributed to the 'if'
line).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215766 91177308-0d34-0410-b5e6-96231b3b80d8