[DebugInfo] Simplify GSYM::AddressRange and GSYM::AddressRanges

Delete unnecessary getters of AddressRange.
Simplify AddressRange::size(): Start <= End check should be checked in an upper layer.
Delete isContiguousWith() that doesn't make sense.
Simplify AddressRanges::insert. Delete commented code. Fix it when more than 1 ranges are to be deleted.
Delete trailing newline.

llvm-svn: 364637