Extend builtin "attribute" syntax to include a notation for

printf-like functions, both builtin functions and those in the
C library. The function-call checker now queries this attribute do
determine if we have a printf-like function, rather than scanning
through the list of "known functions IDs". However, there are 5
functions they are not yet "builtins", so the function-call checker
handles them specifically still:

  - fprintf and vfprintf: the builtins mechanism cannot (yet)
    express FILE* arguments, so these can't be encoded.
  - NSLog: the builtins mechanism cannot (yet) express NSString*
    arguments, so this (and NSLogv) can't be encoded.
  - asprintf and vasprintf: these aren't part of the C99 standard
    library, so we really shouldn't be defining them as builtins in
    the general case (and we don't seem to have the machinery to make
    them builtins only on certain targets and depending on whether
    extensions are enabled).

llvm-svn: 64512