Made a bunch of cleanups, as per Chris' recommendations:

* Removed unused global and member variables
* Fixed comments (CodeGeneratorBug.cpp)
* Check for possibly failing GCC::create() and CBE::create()
* Remove generated files after diffing the output (e.g., shared object)
* Instead of using std::for_each, use explicit loops as std::for_each may
  duplicate the functor, and ours carries state
* Changed member var from cl::opt<std::string> to just std::string
* Fixed doxygen comments
* Fixed string comparisons to use [ str.empty() ] instead of [ str == "" ]
* Cache instances of CBE and GCC in BugDriver across compilations and executions
  while testing tools.

llvm-svn: 7302