[gn build] Add template for running llvm-tblgen and use it to add build file for llvm/lib/IR.

Also adds a boring build file for llvm/lib/BinaryFormat (needed by llvm/lib/IR).

lib/IR marks Attributes and IntrinsicsEnum as public_deps (because IR's public
headers include the generated .inc files), so projects depending on lib/IR will
implicitly depend on them being generated. As a consequence, most targets won't
have to explicitly list a dependency on these tablegen steps (contrast with
intrinsics_gen in the cmake build).

This doesn't yet have the optimization where tablegen's output is only updated
if it's changed.

Differential Revision: https://reviews.llvm.org/D55028#inline-486755

llvm-svn: 347927