Simplify HandleFormatAttr.

 - I have this crazy dream that one day someone will invent a miraculous tool so
   that developers, instead of hand optimizing their source code to obscure its
   intent and decrease its maleability, will instead write what they mean, and
   this strange and wonderful tool -- which I imagine would be called something
   fancy sounding like "an optimizing compiler" -- will make their code fast
   *for* them. With all the saved time, developers could maybe even focus on
   making the magic "optimizing compiler" better!!

 - No intended functionality change, all though I expect the universe to mock me
   for snarkiness.

llvm-svn: 84390