DwarfAccelTable: Fix handling of hash collisions.

It turns out accelerator tables where totally broken if they contained
entries with colliding hashes. The failure mode is pretty bad, as it not
only impacted the colliding entries, but would basically make all the
entries after the first hash collision pointing in the wrong place.

The testcase uses the symbol names that where found to collide during a
clang build.

From a performance point of view, the patch adds a sort and a linear
walk over each bucket contents. While it has a measurable impact on the
accelerator table emission, it's not showing up significantly in clang
profiles (and I'd argue that correctness is priceless :-)).

llvm-svn: 231732