vim: add `returned` keyword

The `returned` keyword was added in SVN r179925.  Update the vim syntax
rules.

llvm-svn: 294808
diff --git a/llvm/utils/vim/syntax/llvm.vim b/llvm/utils/vim/syntax/llvm.vim
index 3a2b828..ee76820 100644
--- a/llvm/utils/vim/syntax/llvm.vim
+++ b/llvm/utils/vim/syntax/llvm.vim
@@ -123,6 +123,7 @@
       \ readnone
       \ readonly
       \ release
+      \ returned
       \ returns_twice
       \ sanitize_address
       \ sanitize_memory