constify the getters in SDNodeDbgValue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219627 91177308-0d34-0410-b5e6-96231b3b80d8
Add an assertion about the integrity of the iterator.
Broken parent scope pointers in inlined DIVariables can cause
ensureAbstractVariableIsCreated to insert new abstract scopes, thus
invalidating the iterator in this loop and leading to hard-to-debug
crashes. Useful when manually reducing IR for testcases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219628 91177308-0d34-0410-b5e6-96231b3b80d8