vim: add `comdat` keyword

The attribute may be applied to a function.  Highlight it as a keyword.

llvm-svn: 284269
diff --git a/llvm/utils/vim/syntax/llvm.vim b/llvm/utils/vim/syntax/llvm.vim
index 1412d6c..4756882 100644
--- a/llvm/utils/vim/syntax/llvm.vim
+++ b/llvm/utils/vim/syntax/llvm.vim
@@ -62,6 +62,7 @@
       \ ccc
       \ cleanup
       \ coldcc
+      \ comdat
       \ common
       \ constant
       \ datalayout