Optimize address range coalescing.

DWARFDebugAranges::Sort() calls std::stable_sort() over a set of address ranges
and then proceeds to collapse neighboring ranges together.

One problem with the current implementation is that it does an incomplete job.
When a pair of ranges are merged the next pair considered does not include the
just-merged range.  IOW, three consecutive ranges are never collapsed into one.

Another problem is that for each range merged we are calling
std::vector::erase() which "shifts" all remaining elements of the vector by one
position on every merge.  The end result (in the worst case) is a quadratic
algorithm -- not good when the input vector is large.

The following patch merges all consecutive ranges and removes the quadratic
behavior.  The implementation uses an auxiliary vector of indices in order to
remember all ranges that can be dropped, then performs the coalescing of ranges
in a single pass.

Patch from Stephen Wilson with some minor modification by me.

llvm-svn: 129595