[lldb][NFC] Allow range-based for-loops on VariableList

Summary:
Adds support for doing range-based for-loops on LLDB's VariableList and
modernises all the index-based for-loops in LLDB where possible.

Reviewers: labath, jdoerfert

Reviewed By: labath

Subscribers: JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70668