[gn build] Add build file for clang/lib/Basic and dependencies

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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349677 91177308-0d34-0410-b5e6-96231b3b80d8
15 files changed