Fix off-by-one in llvm::Format::print.

 - This also shortens the Format.h implementation, and uses the print buffer
   fully (it was wasting a character).

 - This manifested as llvm-test failures, because one side effect was that
   raw_ostream would write garbage '\x00' values into the output stream if it
   happened that the string was at the end of the buffer. This meant that grep
   would report 'Binary file matches', which meant the silly pattern matching
   llvm-test eventually does would fail. Cute. :)

llvm-svn: 79862