vim: add returned keyword

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

llvm-svn: 294808

utils/vim/syntax/llvm.vim

@@ -123,6 +123,7 @@ syn keyword llvmKeyword
       \ readnone
       \ readonly
       \ release
+      \ returned
       \ returns_twice
       \ sanitize_address
       \ sanitize_memory