Always say "the test program", part 2.

Always say "the test program", part 2.
Refer to the various "modules" of bugpoint as "debuggers", not
 "debugging modes" or "modules". I think this is more evocative of
 how they typically act, and it's shorter.
Add a note about how a linker bug can crash bugpoint early.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@9257 91177308-0d34-0410-b5e6-96231b3b80d8