[DebugInfo] Dwarfv5: Avoid unnecessary base_address specifiers in rnglists

Since DWARFv5 rnglists are self descriptive and have distinct encodings
for base-relative (offset_pair) and absolute (start_length) entries,
there's no need to use a base address specifier when describing a lone
address range in a section.

Use that, and improve the test coverage a bit here to include cases like
this and others.

llvm-svn: 337411