vim-patch:8.1.1593: filetype not detected for C++ header files without extension

Problem:    Filetype not detected for C++ header files without extension.
Solution:   Recognize the file by the Emacs file mode. (Dmitry Ilyin,
            closes vim/vim#4593)
6a7af8e2db

runtime/scripts.vim

@@ -212,6 +212,11 @@ else
   elseif s:line1 =~# '^" *[vV]im$'
     set ft=vim
 
+  " libcxx and libstdc++ standard library headers like "iostream" do not have
+  " an extension, recognize the Emacs file mode.
+  elseif s:line1 =~? '-\*-.*C++.*-\*-'
+    set ft=cpp
+
   " MOO
   elseif s:line1 =~# '^\*\* LambdaMOO Database, Format Version \%([1-3]\>\)\@!\d\+ \*\*$'
     set ft=moo

src/nvim/testdir/test_filetype.vim

@@ -581,6 +581,8 @@ let s:script_checks = {
       \ 'cfengine': [['#!/path/cfengine']],
       \ 'erlang': [['#!/path/escript']],
       \ 'haskell': [['#!/path/haskell']],
+      \ 'cpp': [['// Standard iostream objects -*- C++ -*-'],
+      \         ['// -*- C++ -*-']],
       \ }
 
 func Test_script_detection()