Make the ProgramName variable a std::string so we can eliminate the path

portion fo the program name via sys::Path().getLast(). This makes error
messages more readable since this is invariably used only in error
messages.  Instead of:
  /path/to/llvm/bin/directory/toolname: error message
we will now get:
  toolname: error message
Also, since we always have a program name (even if its defaulted), don't
check to see if it is set or not when generating error messages. This
eliminates a bunch of constant strings, saving a little under 1K of data.

llvm-svn: 29842