DwarfAccelTable: remove unneeded bucket terminators.

Last commit fixed the handling of hash collisions, but it introdcuced
unneeded bucket terminators in some places. The generated table was
correct, it can just be a tiny bit smaller. As the previous table was
correct, the test doesn't need updating. If we really wanted to test
this, I could add the section size to the dwarf dump and test for a
precise value there. IMO the correctness test is sufficient.

llvm-svn: 231748