[gn build] Add enough build files to be able to build llvm-tblgen.

Adds build files for:

- llvm/lib/DebugInfo/CodeView
- llvm/lib/DebugInfo/MSF
- llvm/lib/MC
- llvm/lib/TableGen
- llvm/utils/TableGen

All the build files just list sources and deps and are uninteresting.

Differential Revision: https://reviews.llvm.org/D54931

llvm-svn: 347702