This commit modifies the AsmPrinter to avoid emitting any zero-sized symbols to
the .debug_aranges table. Such symbols violate the DWARF 5 spec, which
Each descriptor is a triple consisting of a segment selector, the beginning
address within that segment of a range of text or data covered by some entry
owned by the corresponding compilation unit, followed by the non-zero length
of that range.
In practice, these zero-sized entries produce annoying warnings in lld and
cause GNU binutils to truncate the table when parsing it.
Other parts of LLVM, such as DWARFDebugARanges in the DebugInfo module
(specifically the appendRange method), already avoid emitting zero-sized
symbols to .debug_aranges, but not comprehensively in the AsmPrinter. In
fact, the AsmPrinter does try to avoid emitting such zero-sized symbols when
labels aren't involved, but doesn't when the symbol to emitted is a difference
of two labels; this patch extends that logic to handle the case in which the
symbol is defined via labels.