Remove DwarfDebug::PrevSection, PrevCU is sufficient for handling address range holes.

PrevSection/PrevCU are used to detect holes in the address range of a CU
to ensure the DW_AT_ranges does not include those holes. When we see a
function with no debug info, though it may be in the same range as the
prior and subsequent functions, there should be a gap in the CU's
ranges. By setting PrevCU to null in that case, the range would not be
extended to cover the gap.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217466 91177308-0d34-0410-b5e6-96231b3b80d8
Sink PrevCU updating into DwarfUnit::addRange to ensure consistency

So that the two operations in DwarfDebug couldn't get separated (because
I accidentally separated them in some work in progress), put them
together. While we're here, move DwarfUnit::addRange to
DwarfCompileUnit, since it's not relevant to type units.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217468 91177308-0d34-0410-b5e6-96231b3b80d8