Some simple cleanups:

1. When srcdir == objdir have "spotless" say that it isn't supported in
   that mode rather than just let make say "no such target"
2. Minor doc cleanups
3. Fix the double rebuild problem with yacc files. A missing dependency
   caused parallel builds to skip building the .cpp file after the .cpp
   file was regenerated by bison.

llvm-svn: 24924