vim-patch:9.1.0769: filetype: MLIR files are not recognized

Problem:  filetype: MLIR files are not recognized
Solution: Detect '*.mlir' files as mlir filetype,
          include a mlir filetype plugin
          (Wu, Zhenyu)

closes: vim/vim#15826

347d43bd33

Co-authored-by: Wu, Zhenyu <wuzhenyu@ustc.edu>

runtime/ftplugin/mlir.vim

@@ -0,0 +1,10 @@
+" Vim filetype plugin file
+" Language:	MLIR
+
+if exists("b:did_ftplugin") | finish | endif
+let b:did_ftplugin = 1
+
+setl comments=:///,://
+setl commentstring=//\ %s
+
+let b:undo_ftplugin = "setl commentstring< comments<"