Add LLVM support (#376)

ftplugin/tablegen.vim

@@ -0,0 +1,16 @@
+if exists('g:polyglot_disabled') && index(g:polyglot_disabled, 'llvm') != -1
+  finish
+endif
+
+" Vim filetype plugin file
+" Language: LLVM TableGen
+" Maintainer: The LLVM team, http://llvm.org/
+
+if exists("b:did_ftplugin")
+  finish
+endif
+let b:did_ftplugin = 1
+
+setlocal matchpairs+=<:>
+setlocal softtabstop=2 shiftwidth=2
+setlocal expandtab