Add TableGen ctags(1) emitter and helper script.

To use this in conjunction with exuberant ctags to generate a single
combined tags file, run tblgen first and then
  $ ctags --append [...]

Since some identifiers have corresponding definitions in C++ code,
it can be useful (if using vim) to also use cscope, and
  :set cscopetagorder=1
so that
  :tag X
will preferentially select the tablegen symbol, while
  :cscope find g X
will always find the C++ symbol.

Patch by Kevin Schoedel!

(a couple small formatting changes courtesy of clang-format)

llvm-svn: 177682