[gn build] Add build file for clang/lib/Basic and dependencies, 2nd try

Adds a build file for clang-tblgen and an action for running it, and uses that
to process all the .td files in include/clang/Basic.

Also adds an action to write include/clang/Config/config.h and
include/clang/Basic/Version.inc.

Differential Revision: https://reviews.llvm.org/D55847

(The previous commit of this contained unrelated changes, so I reverted the
whole previous commit and I'm now landing only what I intended to land.)

llvm-svn: 349679