It was raised in D36097 thread that we currently do not have
testcases for section index API in libDebugInfo.
Idea was to add some useful functionality for any tool,
like llvm-dwarfdump which will use that API and require testcases.
Patch introduces next changes:
- Teaches dumper of .debug_ranges to render range section index.
- Teaches dumper about address selection entry and how to render entries properly if section contains them. (previously it did not show result range properly, because did not add base address to result).
- Cosmetic: adds nice header.