DebugInfo: Scope for condition variables more narrowly than the loop variable.

for loops introduce two scopes - one for the outer loop variable and its
initialization, and another for the body of the loop, including any
variable declared inside the loop condition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216288 91177308-0d34-0410-b5e6-96231b3b80d8